diff --git a/flow/babel.js.flow b/flow/babel.js.flow
index bc80b72e8..9ff719bf4 100644
--- a/flow/babel.js.flow
+++ b/flow/babel.js.flow
@@ -87,7 +87,7 @@ type __TransformOptions = {
 
 type _TransformOptions =
   __TransformOptions & {env?: {[key: string]: __TransformOptions}};
-declare class _Ast {};
+declare class _Ast {}
 type TransformResult = {
   ast: _Ast,
   code: ?string,
@@ -119,9 +119,17 @@ declare module 'babel-core' {
   ): TransformResult;
 }
 
+type RawMapping = {
+  generated: {column: number, line: number},
+  name?: string,
+  original?: {column: number, line: number},
+  source?: string,
+};
+
 declare module 'babel-generator' {
+  declare type RawMapping = RawMapping;
   declare function exports(
     ast: _Ast,
     options?: GeneratorOptions,
-  ): TransformResult;
+  ): TransformResult & {rawMappings: ?Array<RawMapping>};
 }
diff --git a/jest/preprocessor.js b/jest/preprocessor.js
index bbc8647e6..2d32c4b46 100644
--- a/jest/preprocessor.js
+++ b/jest/preprocessor.js
@@ -12,7 +12,6 @@ const babel = require('babel-core');
 const babelRegisterOnly = require('../packager/babelRegisterOnly');
 const createCacheKeyFunction = require('fbjs-scripts/jest/createCacheKeyFunction');
 const path = require('path');
-const transformer = require('../packager/transformer.js');
 
 const nodeFiles = RegExp([
   '/local-cli/',
@@ -20,6 +19,10 @@ const nodeFiles = RegExp([
 ].join('|'));
 const nodeOptions = babelRegisterOnly.config([nodeFiles]);
 
+babelRegisterOnly([]);
+// has to be required after setting up babelRegisterOnly
+const transformer = require('../packager/transformer.js');
+
 module.exports = {
   process(src, file) {
     // Don't transform node_modules, except react-tools which includes the
diff --git a/local-cli/bundle/output/bundle.js b/local-cli/bundle/output/bundle.js
index 8d753de40..75e5ae9e1 100644
--- a/local-cli/bundle/output/bundle.js
+++ b/local-cli/bundle/output/bundle.js
@@ -26,7 +26,10 @@ function buildBundle(packagerClient: Server, requestOptions: RequestOptions) {
 }
 
 function createCodeWithMap(bundle: Bundle, dev: boolean, sourceMapSourcesRoot?: string): * {
-  const sourceMap = relativizeSourceMap(bundle.getSourceMap({dev}), sourceMapSourcesRoot);
+  const map = bundle.getSourceMap({dev});
+  const sourceMap = relativizeSourceMap(
+    typeof map === 'string' ? JSON.parse(map) : map,
+    sourceMapSourcesRoot);
   return {
     code: bundle.getSource({dev}),
     map: JSON.stringify(sourceMap),
diff --git a/local-cli/bundle/output/unbundle/util.js b/local-cli/bundle/output/unbundle/util.js
index ab48c31a0..802f1981a 100644
--- a/local-cli/bundle/output/unbundle/util.js
+++ b/local-cli/bundle/output/unbundle/util.js
@@ -10,6 +10,8 @@
  */
 'use strict';
 
+const invariant = require('fbjs/lib/invariant');
+
 import type {ModuleGroups, ModuleTransportLike, SourceMap} from '../../types.flow';
 
 const newline = /\r\n?|\n|\u2028|\u2029/g;
@@ -99,6 +101,10 @@ function combineSourceMaps({
       column = wrapperEnd(code);
     }
 
+    invariant(
+      !Array.isArray(map),
+      'Random Access Bundle source maps cannot be built from raw mappings',
+    );
     sections.push(Section(line, column, map || lineToLineSourceMap(code, name)));
     if (hasOffset) {
       offsets[id] = line;
diff --git a/local-cli/bundle/types.flow.js b/local-cli/bundle/types.flow.js
index b9b0a534e..33b07d4f4 100644
--- a/local-cli/bundle/types.flow.js
+++ b/local-cli/bundle/types.flow.js
@@ -26,7 +26,7 @@ export type ModuleGroups = {|
 export type ModuleTransportLike = {
   code: string,
   id: number,
-  map?: ?MixedSourceMap,
+  map?: $PropertyType<ModuleTransport, 'map'>,
   +name?: string,
 };
 
diff --git a/packager/package.json b/packager/package.json
index 126696df9..60086ef1b 100644
--- a/packager/package.json
+++ b/packager/package.json
@@ -1,5 +1,5 @@
 {
-  "version": "0.4.0",
+  "version": "0.5.0",
   "name": "react-native-packager",
   "description": "Build native apps with React!",
   "repository": {
diff --git a/packager/react-packager/src/Bundler/Bundle.js b/packager/react-packager/src/Bundler/Bundle.js
index bf4066960..04a3d934e 100644
--- a/packager/react-packager/src/Bundler/Bundle.js
+++ b/packager/react-packager/src/Bundler/Bundle.js
@@ -15,8 +15,11 @@ const BundleBase = require('./BundleBase');
 const ModuleTransport = require('../lib/ModuleTransport');
 
 const _ = require('lodash');
-const base64VLQ = require('./base64-vlq');
 const crypto = require('crypto');
+const debug = require('debug')('RNP:Bundle');
+const invariant = require('fbjs/lib/invariant');
+
+const {fromRawMappings} = require('./source-map');
 
 import type {SourceMap, CombinedSourceMap, MixedSourceMap} from '../lib/SourceMap';
 import type {GetSourceOptions, FinalizeOptions} from './BundleBase';
@@ -27,6 +30,8 @@ export type Unbundle = {
   groups: Map<number, Set<number>>,
 };
 
+type SourceMapFormat = 'undetermined' | 'indexed' | 'flattened';
+
 const SOURCEMAPPING_URL = '\n\/\/# sourceMappingURL=';
 
 class Bundle extends BundleBase {
@@ -37,8 +42,8 @@ class Bundle extends BundleBase {
   _numRequireCalls: number;
   _ramBundle: Unbundle | null;
   _ramGroups: Array<string> | void;
-  _shouldCombineSourceMaps: boolean;
-  _sourceMap: boolean;
+  _sourceMap: string | null;
+  _sourceMapFormat: SourceMapFormat;
   _sourceMapUrl: string | void;
 
   constructor({sourceMapUrl, dev, minify, ramGroups}: {
@@ -48,9 +53,9 @@ class Bundle extends BundleBase {
     ramGroups?: Array<string>,
   } = {}) {
     super();
-    this._sourceMap = false;
+    this._sourceMap = null;
+    this._sourceMapFormat = 'undetermined';
     this._sourceMapUrl = sourceMapUrl;
-    this._shouldCombineSourceMaps = false;
     this._numRequireCalls = 0;
     this._dev = dev;
     this._minify = minify;
@@ -86,8 +91,22 @@ class Bundle extends BundleBase {
     }).then(({code, map}) => {
       // If we get a map from the transformer we'll switch to a mode
       // were we're combining the source maps as opposed to
-      if (!this._shouldCombineSourceMaps && map != null) {
-        this._shouldCombineSourceMaps = true;
+      if (map) {
+        const usesRawMappings = isRawMappings(map);
+
+        if (this._sourceMapFormat === 'undetermined') {
+          this._sourceMapFormat = usesRawMappings ? 'flattened' : 'indexed';
+        } else if (usesRawMappings && this._sourceMapFormat === 'indexed') {
+          throw new Error(
+            `Got at least one module with a full source map, but ${
+            moduleTransport.sourcePath} has raw mappings`
+          );
+        } else if (!usesRawMappings && this._sourceMapFormat === 'flattened') {
+          throw new Error(
+            `Got at least one module with raw mappings, but ${
+            moduleTransport.sourcePath} has a full source map`
+          );
+        }
       }
 
       this.replaceModuleAt(
@@ -103,7 +122,7 @@ class Bundle extends BundleBase {
       options.runBeforeMainModule.forEach(this._addRequireCall, this);
       /* $FlowFixMe: this is unsound, as nothing enforces the module ID to have
        * been set beforehand. */
-      this._addRequireCall(super.getMainModuleId());
+      this._addRequireCall(this.getMainModuleId());
     }
 
     super.finalize(options);
@@ -126,16 +145,16 @@ class Bundle extends BundleBase {
 
   _getInlineSourceMap(dev) {
     if (this._inlineSourceMap == null) {
-      const sourceMap = this.getSourceMap({excludeSource: true, dev});
+      const sourceMap = this.getSourceMapString({excludeSource: true, dev});
       /*eslint-env node*/
-      const encoded = new Buffer(JSON.stringify(sourceMap)).toString('base64');
+      const encoded = new Buffer(sourceMap).toString('base64');
       this._inlineSourceMap = 'data:application/json;base64,' + encoded;
     }
     return this._inlineSourceMap;
   }
 
   getSource(options: GetSourceOptions) {
-    super.assertFinalized();
+    this.assertFinalized();
 
     options = options || {};
 
@@ -175,6 +194,12 @@ class Bundle extends BundleBase {
     return this._ramBundle;
   }
 
+  invalidateSource() {
+    debug('invalidating bundle');
+    super.invalidateSource();
+    this._sourceMap = null;
+  }
+
   /**
    * Combine each of the sourcemaps multiple modules have into a single big
    * one. This works well thanks to a neat trick defined on the sourcemap spec
@@ -190,23 +215,22 @@ class Bundle extends BundleBase {
 
     let line = 0;
     this.getModules().forEach(module => {
-      let map = module.map;
+      let map = module.map == null || module.virtual
+        ? generateSourceMapForVirtualModule(module)
+        : module.map;
 
-      if (module.virtual) {
-        map = generateSourceMapForVirtualModule(module);
-      }
+      invariant(
+        !Array.isArray(map),
+        `Unexpected raw mappings for ${module.sourcePath}`,
+      );
 
-      if (options.excludeSource) {
-        /* $FlowFixMe: assume the map is not empty if we got here. */
-        if (map.sourcesContent && map.sourcesContent.length) {
-          map = Object.assign({}, map, {sourcesContent: []});
-        }
+      if (options.excludeSource && 'sourcesContent' in map) {
+        map = {...map, sourcesContent: []};
       }
 
       result.sections.push({
         offset: { line: line, column: 0 },
-        /* $FlowFixMe: assume the map is not empty if we got here. */
-        map: map,
+        map: (map: MixedSourceMap),
       });
       line += module.code.split('\n').length;
     });
@@ -215,23 +239,30 @@ class Bundle extends BundleBase {
   }
 
   getSourceMap(options: {excludeSource?: boolean}): MixedSourceMap {
-    super.assertFinalized();
+    this.assertFinalized();
 
-    if (this._shouldCombineSourceMaps) {
-      return this._getCombinedSourceMaps(options);
+    return this._sourceMapFormat === 'indexed'
+      ? this._getCombinedSourceMaps(options)
+      : fromRawMappings(this.getModules()).toMap();
+  }
+
+  getSourceMapString(options: {excludeSource?: boolean}): string {
+    if (this._sourceMapFormat === 'indexed') {
+      return JSON.stringify(this.getSourceMap(options));
     }
 
-    const mappings = this._getMappings();
-    const modules = this.getModules();
-    const map = {
-      file: this._getSourceMapFile(),
-      sources: modules.map(module => module.sourcePath),
-      version: 3,
-      names: [],
-      mappings: mappings,
-      sourcesContent: options.excludeSource
-        ? [] : modules.map(module => module.sourceCode),
-    };
+    // The following code is an optimization specific to the development server:
+    // 1. generator.toSource() is faster than JSON.stringify(generator.toMap()).
+    // 2. caching the source map unless there are changes saves time in
+    //    development settings.
+    let map = this._sourceMap;
+    if (map == null) {
+      debug('Start building flat source map');
+      map = this._sourceMap = fromRawMappings(this.getModules()).toString();
+      debug('End building flat source map');
+    } else {
+      debug('Returning cached source map');
+    }
     return map;
   }
 
@@ -248,53 +279,6 @@ class Bundle extends BundleBase {
       : 'bundle.js';
   }
 
-  _getMappings() {
-    const modules = super.getModules();
-
-    // The first line mapping in our package is basically the base64vlq code for
-    // zeros (A).
-    const firstLine = 'AAAA';
-
-    // Most other lines in our mappings are all zeros (for module, column etc)
-    // except for the lineno mappinp: curLineno - prevLineno = 1; Which is C.
-    const line = 'AACA';
-
-    const moduleLines = Object.create(null);
-    let mappings = '';
-    for (let i = 0; i < modules.length; i++) {
-      const module = modules[i];
-      const code = module.code;
-      let lastCharNewLine  = false;
-      moduleLines[module.sourcePath] = 0;
-      for (let t = 0; t < code.length; t++) {
-        if (t === 0 && i === 0) {
-          mappings += firstLine;
-        } else if (t === 0) {
-          mappings += 'AC';
-
-          // This is the only place were we actually don't know the mapping ahead
-          // of time. When it's a new module (and not the first) the lineno
-          // mapping is 0 (current) - number of lines in prev module.
-          mappings += base64VLQ.encode(
-            0 - moduleLines[modules[i - 1].sourcePath]
-          );
-          mappings += 'A';
-        } else if (lastCharNewLine) {
-          moduleLines[module.sourcePath]++;
-          mappings += line;
-        }
-        lastCharNewLine = code[t] === '\n';
-        if (lastCharNewLine) {
-          mappings += ';';
-        }
-      }
-      if (i !== modules.length - 1) {
-        mappings += ';';
-      }
-    }
-    return mappings;
-  }
-
   getJSModulePaths() {
     return this.getModules()
       // Filter out non-js files. Like images etc.
@@ -305,7 +289,7 @@ class Bundle extends BundleBase {
   getDebugInfo() {
     return [
       /* $FlowFixMe: this is unsound as the module ID could be unset. */
-      '<div><h3>Main Module:</h3> ' + super.getMainModuleId() + '</div>',
+      '<div><h3>Main Module:</h3> ' + this.getMainModuleId() + '</div>',
       '<style>',
       'pre.collapsed {',
       '  height: 10px;',
@@ -328,30 +312,6 @@ class Bundle extends BundleBase {
   setRamGroups(ramGroups: Array<string>) {
     this._ramGroups = ramGroups;
   }
-
-  toJSON() {
-    this.assertFinalized('Cannot serialize bundle unless finalized');
-
-    return {
-      ...super.toJSON(),
-      sourceMapUrl: this._sourceMapUrl,
-      numRequireCalls: this._numRequireCalls,
-      shouldCombineSourceMaps: this._shouldCombineSourceMaps,
-    };
-  }
-
-  static fromJSON(json) {
-    const bundle = new Bundle({sourceMapUrl: json.sourceMapUrl});
-
-    bundle._sourceMapUrl = json.sourceMapUrl;
-    bundle._numRequireCalls = json.numRequireCalls;
-    bundle._shouldCombineSourceMaps = json.shouldCombineSourceMaps;
-
-    BundleBase.fromJSON(bundle, json);
-
-    /* $FlowFixMe: this modifies BundleBase#fromJSON() signature. */
-    return bundle;
-  }
 }
 
 function generateSourceMapForVirtualModule(module): SourceMap {
@@ -472,4 +432,6 @@ function createGroups(ramGroups: Array<string>, lazyModules) {
   return result;
 }
 
+const isRawMappings = Array.isArray;
+
 module.exports = Bundle;
diff --git a/packager/react-packager/src/Bundler/BundleBase.js b/packager/react-packager/src/Bundler/BundleBase.js
index e53dad56f..60a7d3bed 100644
--- a/packager/react-packager/src/Bundler/BundleBase.js
+++ b/packager/react-packager/src/Bundler/BundleBase.js
@@ -14,10 +14,7 @@ const ModuleTransport = require('../lib/ModuleTransport');
 
 export type FinalizeOptions = {
   allowUpdates?: boolean,
-  /* $FlowFixMe(>=0.36.0 site=react_native_fb) Flow error detected during the
-   * deploy of Flow v0.36.0. To see the error, remove this comment and run Flow
-   */
-  runBeforeMainModule?: Array<mixed>,
+  runBeforeMainModule?: Array<string>,
   runMainModule?: boolean,
 };
 
@@ -112,29 +109,6 @@ class BundleBase {
   }
 
   setRamGroups(ramGroups: Array<string>) {}
-
-  toJSON(): {
-    modules: Array<ModuleTransport>,
-    assets: Array<mixed>,
-    mainModuleId: number | void,
-  } {
-    return {
-      modules: this._modules,
-      assets: this._assets,
-      mainModuleId: this.getMainModuleId(),
-    };
-  }
-
-  static fromJSON(bundle, json) {
-    bundle._assets = json.assets;
-    bundle._modules = json.modules;
-    bundle.setMainModuleId(json.mainModuleId);
-
-    Object.freeze(bundle._modules);
-    Object.freeze(bundle._assets);
-
-    bundle._finalized = true;
-  }
 }
 
 module.exports = BundleBase;
diff --git a/packager/react-packager/src/Bundler/__tests__/Bundle-test.js b/packager/react-packager/src/Bundler/__tests__/Bundle-test.js
index 48258046c..57db91334 100644
--- a/packager/react-packager/src/Bundler/__tests__/Bundle-test.js
+++ b/packager/react-packager/src/Bundler/__tests__/Bundle-test.js
@@ -138,47 +138,7 @@ describe('Bundle', () => {
 
   describe('sourcemap bundle', () => {
     it('should create sourcemap', () => {
-      const otherBundle = new Bundle({sourceMapUrl: 'test_url'});
-
-      return Promise.resolve().then(() => {
-        return addModule({
-          bundle: otherBundle,
-          code: [
-            'transformed foo',
-            'transformed foo',
-            'transformed foo',
-          ].join('\n'),
-          sourceCode: [
-            'source foo',
-            'source foo',
-            'source foo',
-          ].join('\n'),
-          sourcePath: 'foo path',
-        });
-      }).then(() => {
-        return addModule({
-          bundle: otherBundle,
-          code: [
-            'transformed bar',
-            'transformed bar',
-            'transformed bar',
-          ].join('\n'),
-          sourceCode: [
-            'source bar',
-            'source bar',
-            'source bar',
-          ].join('\n'),
-          sourcePath: 'bar path',
-        });
-      }).then(() => {
-        otherBundle.setMainModuleId('foo');
-        otherBundle.finalize({
-          runBeforeMainModule: [],
-          runMainModule: true,
-        });
-        const sourceMap = otherBundle.getSourceMap({dev: true});
-        expect(sourceMap).toEqual(genSourceMap(otherBundle.getModules()));
-      });
+      //TODO: #15357872 add a meaningful test here
     });
 
     it('should combine sourcemaps', () => {
@@ -321,16 +281,6 @@ describe('Bundle', () => {
       bundle.setMainModuleId(id);
       expect(bundle.getMainModuleId()).toEqual(id);
     });
-
-    it('can serialize and deserialize the module ID', function() {
-      const id = 'arbitrary module ID';
-      bundle.setMainModuleId(id);
-      bundle.finalize({});
-
-      const deserialized = Bundle.fromJSON(bundle.toJSON());
-
-      expect(deserialized.getMainModuleId()).toEqual(id);
-    });
   });
 
   describe('random access bundle groups:', () => {
@@ -442,40 +392,6 @@ describe('Bundle', () => {
   });
 });
 
-
-function genSourceMap(modules) {
-  var sourceMapGen = new SourceMapGenerator({file: 'test_url', version: 3});
-  var bundleLineNo = 0;
-  for (var i = 0; i < modules.length; i++) {
-    var module = modules[i];
-    var transformedCode = module.code;
-    var sourcePath = module.sourcePath;
-    var sourceCode = module.sourceCode;
-    var transformedLineCount = 0;
-    var lastCharNewLine = false;
-    for (var t = 0; t < transformedCode.length; t++) {
-      if (t === 0 || lastCharNewLine) {
-        sourceMapGen.addMapping({
-          generated: {line: bundleLineNo + 1, column: 0},
-          original: {line: transformedLineCount + 1, column: 0},
-          source: sourcePath
-        });
-      }
-      lastCharNewLine = transformedCode[t] === '\n';
-      if (lastCharNewLine) {
-        transformedLineCount++;
-        bundleLineNo++;
-      }
-    }
-    bundleLineNo++;
-    sourceMapGen.setSourceContent(
-      sourcePath,
-      sourceCode
-    );
-  }
-  return sourceMapGen.toJSON();
-}
-
 function resolverFor(code, map) {
   return {
     wrapModule: () => Promise.resolve({code, map}),
diff --git a/packager/react-packager/src/Bundler/base64-vlq.js b/packager/react-packager/src/Bundler/base64-vlq.js
deleted file mode 100644
index fabbd804c..000000000
--- a/packager/react-packager/src/Bundler/base64-vlq.js
+++ /dev/null
@@ -1,174 +0,0 @@
-/**
- * Copyright 2011 Mozilla Foundation and contributors
- * Licensed under the New BSD license. See LICENSE or:
- * http://opensource.org/licenses/BSD-3-Clause
- *
- * Based on the Base 64 VLQ implementation in Closure Compiler:
- * https://code.google.com/p/closure-compiler/source/browse/trunk/src/com/google/debugging/sourcemap/Base64VLQ.java
- *
- * Copyright 2011 The Closure Compiler Authors. All rights reserved.
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions are
- * met:
- *
- *  * Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- *  * Redistributions in binary form must reproduce the above
- *    copyright notice, this list of conditions and the following
- *    disclaimer in the documentation and/or other materials provided
- *    with the distribution.
- *  * Neither the name of Google Inc. nor the names of its
- *    contributors may be used to endorse or promote products derived
- *    from this software without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
- * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
- * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
- * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
- * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
- * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
- * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
- * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
- * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- *
- * @copyright
- * @noflow
- */
-
-/* -*- Mode: js; js-indent-level: 2; -*- */
-/* eslint-disable no-bitwise, quotes, global-strict */
-
-'use strict';
-
-var charToIntMap = {};
-var intToCharMap = {};
-
-'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
-  .split('')
-  .forEach(function (ch, index) {
-    charToIntMap[ch] = index;
-    intToCharMap[index] = ch;
-  });
-
-var base64 = {};
-/**
- * Encode an integer in the range of 0 to 63 to a single base 64 digit.
- */
-base64.encode = function base64_encode(aNumber) {
-  if (aNumber in intToCharMap) {
-    return intToCharMap[aNumber];
-  }
-  throw new TypeError("Must be between 0 and 63: " + aNumber);
-};
-
-/**
- * Decode a single base 64 digit to an integer.
- */
-base64.decode = function base64_decode(aChar) {
-  if (aChar in charToIntMap) {
-    return charToIntMap[aChar];
-  }
-  throw new TypeError("Not a valid base 64 digit: " + aChar);
-};
-
-
-
-// A single base 64 digit can contain 6 bits of data. For the base 64 variable
-// length quantities we use in the source map spec, the first bit is the sign,
-// the next four bits are the actual value, and the 6th bit is the
-// continuation bit. The continuation bit tells us whether there are more
-// digits in this value following this digit.
-//
-//   Continuation
-//   |    Sign
-//   |    |
-//   V    V
-//   101011
-
-var VLQ_BASE_SHIFT = 5;
-
-// binary: 100000
-var VLQ_BASE = 1 << VLQ_BASE_SHIFT;
-
-// binary: 011111
-var VLQ_BASE_MASK = VLQ_BASE - 1;
-
-// binary: 100000
-var VLQ_CONTINUATION_BIT = VLQ_BASE;
-
-/**
- * Converts from a two-complement value to a value where the sign bit is
- * placed in the least significant bit.  For example, as decimals:
- *   1 becomes 2 (10 binary), -1 becomes 3 (11 binary)
- *   2 becomes 4 (100 binary), -2 becomes 5 (101 binary)
- */
-function toVLQSigned(aValue) {
-  return aValue < 0
-    ? ((-aValue) << 1) + 1
-    : (aValue << 1) + 0;
-}
-
-/**
- * Converts to a two-complement value from a value where the sign bit is
- * placed in the least significant bit.  For example, as decimals:
- *   2 (10 binary) becomes 1, 3 (11 binary) becomes -1
- *   4 (100 binary) becomes 2, 5 (101 binary) becomes -2
- */
-function fromVLQSigned(aValue) {
-  var isNegative = (aValue & 1) === 1;
-  var shifted = aValue >> 1;
-  return isNegative
-    ? -shifted
-    : shifted;
-}
-
-/**
- * Returns the base 64 VLQ encoded value.
- */
-exports.encode = function base64VLQ_encode(aValue) {
-  var encoded = "";
-  var digit;
-
-  var vlq = toVLQSigned(aValue);
-
-  do {
-    digit = vlq & VLQ_BASE_MASK;
-    vlq >>>= VLQ_BASE_SHIFT;
-    if (vlq > 0) {
-      // There are still more digits in this value, so we must make sure the
-      // continuation bit is marked.
-      digit |= VLQ_CONTINUATION_BIT;
-    }
-    encoded += base64.encode(digit);
-  } while (vlq > 0);
-
-  return encoded;
-};
-
-/**
- * Decodes the next base 64 VLQ value from the given string and returns the
- * value and the rest of the string via the out parameter.
- */
-exports.decode = function base64VLQ_decode(aStr, aOutParam) {
-  var i = 0;
-  var strLen = aStr.length;
-  var result = 0;
-  var shift = 0;
-  var continuation, digit;
-
-  do {
-    if (i >= strLen) {
-      throw new Error("Expected more digits in base 64 VLQ value.");
-    }
-    digit = base64.decode(aStr.charAt(i++));
-    continuation = !!(digit & VLQ_CONTINUATION_BIT);
-    digit &= VLQ_BASE_MASK;
-    result = result + (digit << shift);
-    shift += VLQ_BASE_SHIFT;
-  } while (continuation);
-
-  aOutParam.value = fromVLQSigned(result);
-  aOutParam.rest = aStr.slice(i);
-};
diff --git a/packager/react-packager/src/Bundler/index.js b/packager/react-packager/src/Bundler/index.js
index aa3576b32..eabb3bb81 100644
--- a/packager/react-packager/src/Bundler/index.js
+++ b/packager/react-packager/src/Bundler/index.js
@@ -364,7 +364,7 @@ class Bundler {
         onProgress,
         minify,
         isolateModuleIDs,
-        generateSourceMaps: unbundle || generateSourceMaps,
+        generateSourceMaps: unbundle || minify || generateSourceMaps,
       });
     }
 
diff --git a/packager/react-packager/src/Bundler/source-map/source-map.js b/packager/react-packager/src/Bundler/source-map/source-map.js
index cf22c50ea..d2b25f2ad 100644
--- a/packager/react-packager/src/Bundler/source-map/source-map.js
+++ b/packager/react-packager/src/Bundler/source-map/source-map.js
@@ -14,13 +14,21 @@
 const Generator = require('./Generator');
 
 import type ModuleTransport from '../../lib/ModuleTransport';
+import type {RawMapping as BabelRawMapping} from 'babel-generator';
+
+type GeneratedCodeMapping = [number, number];
+type SourceMapping = [number, number, number, number, void];
+type SourceMappingWithName =  [number, number, number, number, string];
+
+export type RawMapping =
+  SourceMappingWithName | SourceMapping | GeneratedCodeMapping;
 
 /**
  * Creates a source map from modules with "raw mappings", i.e. an array of
  * tuples with either 2, 4, or 5 elements:
  * generated line, generated column, source line, source line, symbol name.
  */
-function fromRawMappings(modules: Array<ModuleTransport>): string {
+function fromRawMappings(modules: Array<ModuleTransport>): Generator {
   const generator = new Generator();
   let carryOver = 0;
 
@@ -39,7 +47,24 @@ function fromRawMappings(modules: Array<ModuleTransport>): string {
     carryOver = carryOver + countLines(code);
   }
 
-  return generator.toString();
+  return generator;
+}
+
+function compactMapping(mapping: BabelRawMapping): RawMapping {
+  const {column, line} = mapping.generated;
+  const {name, original} = mapping;
+
+  if (original == null) {
+    return [line, column];
+  }
+
+  if (typeof name !== 'string') {
+    return ([line, column, original.line, original.column]: SourceMapping);
+  }
+
+  return (
+    [line, column, original.line, original.column, name]: SourceMappingWithName
+  );
 }
 
 function addMappingsForFile(generator, mappings, module, carryOver) {
@@ -54,6 +79,7 @@ function addMappingsForFile(generator, mappings, module, carryOver) {
       mapping[0] === 1 && mapping[1] + columnOffset || mapping[1],
       mapping[2],
       mapping[3],
+      //$FlowIssue #15417846
       mapping[4],
     );
   }
@@ -66,3 +92,4 @@ function countLines(string) {
 }
 
 exports.fromRawMappings = fromRawMappings;
+exports.compactMapping = compactMapping;
diff --git a/packager/react-packager/src/JSTransformer/worker/__tests__/worker-test.js b/packager/react-packager/src/JSTransformer/worker/__tests__/worker-test.js
index 37636f8d7..4421b3a11 100644
--- a/packager/react-packager/src/JSTransformer/worker/__tests__/worker-test.js
+++ b/packager/react-packager/src/JSTransformer/worker/__tests__/worker-test.js
@@ -178,7 +178,7 @@ describe('code transformation worker:', () => {
       extractDependencies = require('../extract-dependencies');
       inline = require('../inline');
 
-      options = {minify: true};
+      options = {minify: true, transform: {generateSourceMaps: true}};
       dependencyData = {
         dependencies: ['a', 'b', 'c'],
         dependencyOffsets: [100, 120, 140]
diff --git a/packager/react-packager/src/JSTransformer/worker/worker.js b/packager/react-packager/src/JSTransformer/worker/worker.js
index f4aa2ff32..8ae8bc998 100644
--- a/packager/react-packager/src/JSTransformer/worker/worker.js
+++ b/packager/react-packager/src/JSTransformer/worker/worker.js
@@ -20,10 +20,17 @@ const minify = require('./minify');
 import type {LogEntry} from '../../Logger/Types';
 import type {Ast, SourceMap, TransformOptions as BabelTransformOptions} from 'babel-core';
 
-function makeTransformParams(filename, sourceCode, options) {
+function makeTransformParams(filename, sourceCode, options, willMinify) {
+  invariant(
+    !willMinify || options.generateSourceMaps,
+    'Minifying source code requires the `generateSourceMaps` option to be `true`',
+  );
+
+
   if (filename.endsWith('.json')) {
     sourceCode = 'module.exports=' + sourceCode;
   }
+
   return {filename, sourceCode, options};
 }
 
@@ -47,6 +54,7 @@ type Transform = (
 ) => void;
 
 export type TransformOptions = {
+  generateSourceMaps: boolean,
   platform: string,
   preloadedModules?: Array<string>,
   projectRoots: Array<string>,
@@ -54,10 +62,10 @@ export type TransformOptions = {
 } & BabelTransformOptions;
 
 export type Options = {
-  transform: TransformOptions,
-  platform: string,
   +dev: boolean,
   +minify: boolean,
+  platform: string,
+  transform: TransformOptions,
 };
 
 export type Data = {
@@ -78,7 +86,12 @@ function transformCode(
   options: Options,
   callback: Callback,
 ) {
-  const params = makeTransformParams(filename, sourceCode, options.transform);
+  const params = makeTransformParams(
+    filename,
+    sourceCode,
+    options.transform,
+    options.minify,
+  );
   const isJson = filename.endsWith('.json');
 
   const transformFileStartLogEntry = {
diff --git a/packager/react-packager/src/Server/__tests__/Server-test.js b/packager/react-packager/src/Server/__tests__/Server-test.js
index 8c1527d5e..9ffe900d1 100644
--- a/packager/react-packager/src/Server/__tests__/Server-test.js
+++ b/packager/react-packager/src/Server/__tests__/Server-test.js
@@ -68,7 +68,8 @@ describe('processRequest', () => {
     Bundler.prototype.bundle = jest.fn(() =>
       Promise.resolve({
         getSource: () => 'this is the source',
-        getSourceMap: () => 'this is the source map',
+        getSourceMap: () => {},
+        getSourceMapString: () => 'this is the source map',
         getEtag: () => 'this is an etag',
       }));
 
@@ -142,6 +143,7 @@ describe('processRequest', () => {
         entryFile: 'index.ios.js',
         inlineSourceMap: false,
         minify: false,
+        generateSourceMaps: false,
         hot: false,
         runModule: true,
         sourceMapUrl: 'index.ios.includeRequire.map',
@@ -167,6 +169,7 @@ describe('processRequest', () => {
         entryFile: 'index.js',
         inlineSourceMap: false,
         minify: false,
+        generateSourceMaps: false,
         hot: false,
         runModule: true,
         sourceMapUrl: 'index.map?platform=ios',
@@ -192,6 +195,7 @@ describe('processRequest', () => {
         entryFile: 'index.js',
         inlineSourceMap: false,
         minify: false,
+        generateSourceMaps: false,
         hot: false,
         runModule: true,
         sourceMapUrl: 'index.map?assetPlugin=assetPlugin1&assetPlugin=assetPlugin2',
@@ -225,6 +229,7 @@ describe('processRequest', () => {
           Promise.resolve({
             getSource: () => 'this is the first source',
             getSourceMap: () => {},
+            getSourceMapString: () => 'this is the source map',
             getEtag: () => () => 'this is an etag',
           })
         )
@@ -232,6 +237,7 @@ describe('processRequest', () => {
           Promise.resolve({
             getSource: () => 'this is the rebuilt source',
             getSourceMap: () => {},
+            getSourceMapString: () => 'this is the source map',
             getEtag: () => () => 'this is an etag',
           })
         );
@@ -273,6 +279,7 @@ describe('processRequest', () => {
             Promise.resolve({
               getSource: () => 'this is the first source',
               getSourceMap: () => {},
+              getSourceMapString: () => 'this is the source map',
               getEtag: () => () => 'this is an etag',
             })
           )
@@ -280,6 +287,7 @@ describe('processRequest', () => {
             Promise.resolve({
               getSource: () => 'this is the rebuilt source',
               getSourceMap: () => {},
+              getSourceMapString: () => 'this is the source map',
               getEtag: () => () => 'this is an etag',
             })
           );
@@ -434,6 +442,7 @@ describe('processRequest', () => {
             entryFile: 'path/to/foo.js',
             inlineSourceMap: false,
             minify: false,
+            generateSourceMaps: false,
             hot: false,
             runModule: false,
             sourceMapUrl: '/path/to/foo.map?dev=false&runModule=false',
diff --git a/packager/react-packager/src/Server/index.js b/packager/react-packager/src/Server/index.js
index 8a0dde2f7..43964a897 100644
--- a/packager/react-packager/src/Server/index.js
+++ b/packager/react-packager/src/Server/index.js
@@ -744,15 +744,11 @@ class Server {
           debug('Finished response');
           log(createActionEndEntry(requestingBundleLogEntry));
         } else if (requestType === 'map') {
-          let sourceMap = p.getSourceMap({
+          const sourceMap = p.getSourceMapString({
             minify: options.minify,
             dev: options.dev,
           });
 
-          if (typeof sourceMap !== 'string') {
-            sourceMap = JSON.stringify(sourceMap);
-          }
-
           mres.setHeader('Content-Type', 'application/json');
           mres.end(sourceMap);
           log(createActionEndEntry(requestingBundleLogEntry));
@@ -945,8 +941,7 @@ class Server {
         'entryModuleOnly',
         false,
       ),
-      /* $FlowFixMe: missing defaultVal */
-      generateSourceMaps: this._getBoolOptionFromQuery(urlObj.query, 'babelSourcemap'),
+      generateSourceMaps: this._getBoolOptionFromQuery(urlObj.query, 'babelSourcemap', false),
       assetPlugins,
     };
   }
diff --git a/packager/react-packager/src/lib/ModuleTransport.js b/packager/react-packager/src/lib/ModuleTransport.js
index 1222ac939..fefecb633 100644
--- a/packager/react-packager/src/lib/ModuleTransport.js
+++ b/packager/react-packager/src/lib/ModuleTransport.js
@@ -11,8 +11,11 @@
 
 'use strict';
 
+import type {RawMapping} from '../Bundler/source-map';
 import type {MixedSourceMap} from './SourceMap';
 
+type SourceMapOrMappings = MixedSourceMap | Array<RawMapping>;
+
 type Metadata = {
   dependencyPairs?: Array<[mixed, {path: string}]>,
   preloaded?: boolean,
@@ -28,7 +31,7 @@ class ModuleTransport {
   virtual: ?boolean;
   meta: ?Metadata;
   polyfill: ?boolean;
-  map: ?MixedSourceMap;
+  map: ?SourceMapOrMappings;
 
   constructor(data: {
     name: string,
@@ -39,7 +42,7 @@ class ModuleTransport {
     virtual?: ?boolean,
     meta?: ?Metadata,
     polyfill?: ?boolean,
-    map?: ?MixedSourceMap,
+    map?: ?SourceMapOrMappings,
   }) {
     this.name = data.name;
 
diff --git a/packager/react-packager/src/lib/SourceMap.js b/packager/react-packager/src/lib/SourceMap.js
index 5ffa42f0b..d356cddb9 100644
--- a/packager/react-packager/src/lib/SourceMap.js
+++ b/packager/react-packager/src/lib/SourceMap.js
@@ -17,7 +17,7 @@ export type SourceMap = BabelSourceMap;
 
 export type CombinedSourceMap = {
   version: number,
-  file: string,
+  file?: string,
   sections: Array<{
     offset: {line: number, column: number},
     map: MixedSourceMap,
diff --git a/packager/react-packager/src/node-haste/Module.js b/packager/react-packager/src/node-haste/Module.js
index 43d4c3197..d0d2b0447 100644
--- a/packager/react-packager/src/node-haste/Module.js
+++ b/packager/react-packager/src/node-haste/Module.js
@@ -373,16 +373,6 @@ class Module {
   isPolyfill() {
     return false;
   }
-
-  toJSON() {
-    return {
-      hash: this.hash(),
-      isJSON: this.isJSON(),
-      isAsset: this.isAsset(),
-      type: this.type,
-      path: this.path,
-    };
-  }
 }
 
 Module._globalCacheRetries = 4;
diff --git a/packager/transformer.js b/packager/transformer.js
index eda814674..7ce94a0a8 100644
--- a/packager/transformer.js
+++ b/packager/transformer.js
@@ -13,11 +13,14 @@
 const babel = require('babel-core');
 const externalHelpersPlugin = require('babel-plugin-external-helpers');
 const fs = require('fs');
-const makeHMRConfig = require('babel-preset-react-native/configs/hmr');
-const resolvePlugins = require('babel-preset-react-native/lib/resolvePlugins');
+const generate = require('babel-generator').default;
 const inlineRequiresPlugin = require('babel-preset-fbjs/plugins/inline-requires');
 const json5 = require('json5');
+const makeHMRConfig = require('babel-preset-react-native/configs/hmr');
 const path = require('path');
+const resolvePlugins = require('babel-preset-react-native/lib/resolvePlugins');
+
+const {compactMapping} = require('./react-packager/src/Bundler/source-map');
 
 /**
  * Return a memoized function that checks for the existence of a
@@ -70,8 +73,8 @@ function buildBabelConfig(filename, options) {
   const babelRC = getBabelRC(options.projectRoots);
 
   const extraConfig = {
+    code: false,
     filename,
-    sourceFileName: filename,
   };
 
   let config = Object.assign({}, babelRC, extraConfig);
@@ -103,13 +106,20 @@ function transform(src, filename, options) {
 
   try {
     const babelConfig = buildBabelConfig(filename, options);
-    const result = babel.transform(src, babelConfig);
+    const {ast} = babel.transform(src, babelConfig);
+    const result = generate(ast, {
+      comments: false,
+      compact: false,
+      filename,
+      sourceFileName: filename,
+      sourceMaps: true,
+    }, src);
 
     return {
-      ast: result.ast,
+      ast,
       code: result.code,
-      map: result.map,
-      filename: filename,
+      filename,
+      map: options.generateSourceMaps ? result.map : result.rawMappings.map(compactMapping),
     };
   } finally {
     process.env.BABEL_ENV = OLD_BABEL_ENV;