diff --git a/packages/metro-bundler/src/JSTransformer/index.js b/packages/metro-bundler/src/JSTransformer/index.js
index 5d5a83da..6f44ed48 100644
--- a/packages/metro-bundler/src/JSTransformer/index.js
+++ b/packages/metro-bundler/src/JSTransformer/index.js
@@ -19,7 +19,7 @@ const Worker = require('jest-worker').default;
 
 import type {Options, TransformedCode} from './worker';
 import type {LocalPath} from '../node-haste/lib/toLocalPath';
-import type {RawMappings} from '../lib/SourceMap';
+import type {MappingsMap} from '../lib/SourceMap';
 import type {ResultWithMap} from './worker/minify';
 
 import typeof {
@@ -78,7 +78,7 @@ module.exports = class Transformer {
   async minify(
     filename: string,
     code: string,
-    sourceMap: RawMappings,
+    sourceMap: MappingsMap,
   ): Promise<ResultWithMap> {
     return await this._worker.minify(filename, code, sourceMap);
   }
diff --git a/packages/metro-bundler/src/JSTransformer/worker/index.js b/packages/metro-bundler/src/JSTransformer/worker/index.js
index caf97e8e..0bebcce7 100644
--- a/packages/metro-bundler/src/JSTransformer/worker/index.js
+++ b/packages/metro-bundler/src/JSTransformer/worker/index.js
@@ -12,7 +12,6 @@
 
 'use strict';
 
-const asyncify = require('async/asyncify');
 const constantFolding = require('./constant-folding');
 const extractDependencies = require('./extract-dependencies');
 const inline = require('./inline');
@@ -34,7 +33,7 @@ export type TransformedCode = {
   code: string,
   dependencies: Array<string>,
   dependencyOffsets: Array<number>,
-  map?: ?CompactRawMappings,
+  map: CompactRawMappings,
 };
 
 export type TransformArgs<ExtraOptions: {}> = {|
@@ -134,7 +133,7 @@ function transformCode(
       : transformed.map;
 
   // Convert the sourcemaps to Compact Raw source maps.
-  const map = rawMappings ? rawMappings.map(compactMapping) : null;
+  const map = rawMappings ? rawMappings.map(compactMapping) : [];
 
   let code = transformed.code;
   if (isJson) {
@@ -168,7 +167,7 @@ function transformCode(
 exports.minify = async function(
   filename: string,
   code: string,
-  sourceMap: RawMappings,
+  sourceMap: MappingsMap,
 ): Promise<ResultWithMap> {
   try {
     return minify.withSourceMap(code, sourceMap, filename);
diff --git a/packages/metro-bundler/src/JSTransformer/worker/minify.js b/packages/metro-bundler/src/JSTransformer/worker/minify.js
index 736dc2f6..41b5b436 100644
--- a/packages/metro-bundler/src/JSTransformer/worker/minify.js
+++ b/packages/metro-bundler/src/JSTransformer/worker/minify.js
@@ -14,17 +14,7 @@
 
 const uglify = require('uglify-es');
 
-const {
-  compactMapping,
-  fromRawMappings,
-  toRawMappings,
-} = require('../../Bundler/source-map');
-
-import type {
-  CompactRawMappings,
-  MappingsMap,
-  RawMappings,
-} from '../../lib/SourceMap';
+import type {MappingsMap} from '../../lib/SourceMap';
 
 export type ResultWithMap = {
   code: string,
@@ -37,15 +27,9 @@ function noSourceMap(code: string): string {
 
 function withSourceMap(
   code: string,
-  sourceMap: ?MappingsMap | RawMappings,
+  sourceMap: ?MappingsMap,
   filename: string,
 ): ResultWithMap {
-  if (sourceMap && Array.isArray(sourceMap)) {
-    sourceMap = fromRawMappings([
-      {code, source: code, map: sourceMap, path: filename},
-    ]).toMap(undefined, {});
-  }
-
   const result = minify(code, sourceMap);
 
   const map: MappingsMap = JSON.parse(result.map);
@@ -53,19 +37,6 @@ function withSourceMap(
   return {code: result.code, map};
 }
 
-function withRawMappings(
-  code: string,
-  map: RawMappings,
-  filename: string,
-): {code: string, map: CompactRawMappings} {
-  const result = withSourceMap(code, map, filename);
-
-  return {
-    code: result.code,
-    map: toRawMappings(result.map).map(compactMapping),
-  };
-}
-
 function minify(inputCode: string, inputMap: ?MappingsMap) {
   const result = uglify.minify(inputCode, {
     mangle: {toplevel: true},
@@ -93,6 +64,5 @@ function minify(inputCode: string, inputMap: ?MappingsMap) {
 
 module.exports = {
   noSourceMap,
-  withRawMappings,
   withSourceMap,
 };
diff --git a/packages/metro-bundler/src/Resolver/index.js b/packages/metro-bundler/src/Resolver/index.js
index 5b1edd69..755fc1e1 100644
--- a/packages/metro-bundler/src/Resolver/index.js
+++ b/packages/metro-bundler/src/Resolver/index.js
@@ -35,11 +35,7 @@ import type {
 } from '../lib/TransformCaching';
 import type {GlobalTransformCache} from '../lib/GlobalTransformCache';
 
-type MinifyCode = (
-  filePath: string,
-  code: string,
-  map: ?MappingsMap,
-) => Promise<{code: string, map: ?MappingsMap}>;
+import typeof {minify as MinifyCode} from '../JSTransformer/worker';
 
 type ContainsTransformerOptions = {+transformer: JSTransformerOptions};
 
@@ -273,7 +269,7 @@ class Resolver {
     );
 
     const minified = await this._minifyCode(path, code, sourceMap);
-    const result = await this._postMinifyProcess(minified);
+    const result = await this._postMinifyProcess({...minified});
 
     return {
       code: result.code,
diff --git a/packages/metro-bundler/src/lib/TransformCaching.js b/packages/metro-bundler/src/lib/TransformCaching.js
index 533cd90a..bddf743f 100644
--- a/packages/metro-bundler/src/lib/TransformCaching.js
+++ b/packages/metro-bundler/src/lib/TransformCaching.js
@@ -35,7 +35,7 @@ export type CachedResult = {
   code: string,
   dependencies: Array<string>,
   dependencyOffsets: Array<number>,
-  map?: ?CompactRawMappings,
+  map: CompactRawMappings,
 };
 
 export type TransformCacheResult = {|
@@ -336,7 +336,7 @@ function readMetadataFileSync(
   cachedSourceHash: string,
   dependencies: Array<string>,
   dependencyOffsets: Array<number>,
-  sourceMap: ?CompactRawMappings,
+  sourceMap: CompactRawMappings,
 } {
   const metadataStr = fs.readFileSync(metadataFilePath, 'utf8');
   const metadata = tryParseJSON(metadataStr);
diff --git a/packages/metro-bundler/src/node-haste/Module.js b/packages/metro-bundler/src/node-haste/Module.js
index 21f7909f..54032547 100644
--- a/packages/metro-bundler/src/node-haste/Module.js
+++ b/packages/metro-bundler/src/node-haste/Module.js
@@ -27,7 +27,7 @@ import type {
   Options as WorkerOptions,
 } from '../JSTransformer/worker';
 import type {GlobalTransformCache} from '../lib/GlobalTransformCache';
-import type {MappingsMap} from '../lib/SourceMap';
+import type {CompactRawMappings} from '../lib/SourceMap';
 import type {
   TransformCache,
   GetTransformCacheKey,
@@ -42,7 +42,7 @@ export type ReadResult = {
   +code: string,
   +dependencies: Array<string>,
   +dependencyOffsets?: ?Array<number>,
-  +map?: ?(MappingsMap | Array<RawMapping>),
+  +map: CompactRawMappings,
   +source: string,
 };