diff --git a/packages/metro-bundler/src/ModuleGraph/node-haste/Module.js b/packages/metro-bundler/src/ModuleGraph/node-haste/Module.js
index d57f14ea..5d361df3 100644
--- a/packages/metro-bundler/src/ModuleGraph/node-haste/Module.js
+++ b/packages/metro-bundler/src/ModuleGraph/node-haste/Module.js
@@ -12,7 +12,7 @@
 'use strict';
 
 import type {CachedReadResult, ReadResult} from '../../node-haste/Module';
-import type {TransformedFile} from '../types.flow';
+import type {TransformedCodeFile} from '../types.flow';
 import type {ModuleCache} from './node-haste.flow';
 
 module.exports = class Module {
@@ -25,7 +25,7 @@ module.exports = class Module {
   constructor(
     path: string,
     moduleCache: ModuleCache,
-    info: TransformedFile,
+    info: TransformedCodeFile,
   ) {
     this.hasteID = Promise.resolve(info.hasteID);
     this.moduleCache = moduleCache;
diff --git a/packages/metro-bundler/src/ModuleGraph/node-haste/ModuleCache.js b/packages/metro-bundler/src/ModuleGraph/node-haste/ModuleCache.js
index 2a483af8..73bda233 100644
--- a/packages/metro-bundler/src/ModuleGraph/node-haste/ModuleCache.js
+++ b/packages/metro-bundler/src/ModuleGraph/node-haste/ModuleCache.js
@@ -14,19 +14,19 @@
 const Module = require('./Module');
 const Package = require('./Package');
 
-import type {PackageData, TransformedFile} from '../types.flow';
+import type {PackageData, TransformedCodeFile} from '../types.flow';
 
 type GetClosestPackageFn = (filePath: string) => ?string;
 
 module.exports = class ModuleCache {
   _getClosestPackage: GetClosestPackageFn;
-  getTransformedFile: string => TransformedFile;
+  getTransformedFile: string => TransformedCodeFile;
   modules: Map<string, Module>;
   packages: Map<string, Package>;
 
   constructor(
     getClosestPackage: GetClosestPackageFn,
-    getTransformedFile: string => TransformedFile,
+    getTransformedFile: string => TransformedCodeFile,
   ) {
     this._getClosestPackage = getClosestPackage;
     this.getTransformedFile = getTransformedFile;
diff --git a/packages/metro-bundler/src/ModuleGraph/node-haste/node-haste.js b/packages/metro-bundler/src/ModuleGraph/node-haste/node-haste.js
index c9ba2468..fd3e48df 100644
--- a/packages/metro-bundler/src/ModuleGraph/node-haste/node-haste.js
+++ b/packages/metro-bundler/src/ModuleGraph/node-haste/node-haste.js
@@ -18,7 +18,7 @@ import type { // eslint-disable-line sort-requires
 
 import type {
   ResolveFn,
-  TransformedFile,
+  TransformedCodeFile,
 } from '../types.flow';
 
 const DependencyGraphHelpers = require('../../node-haste/DependencyGraph/DependencyGraphHelpers');
@@ -34,7 +34,7 @@ const defaults = require('../../../defaults');
 type ResolveOptions = {|
   assetExts: Extensions,
   extraNodeModules: {[id: string]: string},
-  transformedFiles: {[path: Path]: TransformedFile},
+  transformedFiles: {[path: Path]: TransformedCodeFile},
 |};
 
 const platforms = new Set(defaults.platforms);
diff --git a/packages/metro-bundler/src/ModuleGraph/types.flow.js b/packages/metro-bundler/src/ModuleGraph/types.flow.js
index ec447ccb..18def76e 100644
--- a/packages/metro-bundler/src/ModuleGraph/types.flow.js
+++ b/packages/metro-bundler/src/ModuleGraph/types.flow.js
@@ -27,10 +27,10 @@ export type File = {|
   code: string,
   map?: ?Object,
   path: string,
-  type: FileTypes,
+  type: CodeFileTypes,
 |};
 
-type FileTypes = 'module' | 'script' | 'asset';
+type CodeFileTypes = 'module' | 'script';
 
 export type GraphFn = (
   entryPoints: Iterable<string>,
@@ -125,16 +125,31 @@ export type TransformResults = {[string]: TransformResult};
 
 export type TransformVariants = {[key: string]: Object};
 
-export type TransformedFile = {
-  assetContent: ?string,
-  code: string,
-  file: string,
-  hasteID: ?string,
+export type TransformedCodeFile = {
+  +code: string,
+  +file: string,
+  +hasteID: ?string,
   package?: PackageData,
-  transformed: TransformResults,
-  type: FileTypes,
+  +transformed: TransformResults,
+  +type: CodeFileTypes,
 };
 
+export type AssetFile = {|
+  +assetContentBase64: string,
+  +filePath: string,
+|};
+
+export type TransformedSourceFile =
+  | {|
+    +type: 'code',
+    +details: TransformedCodeFile,
+  |}
+  | {|
+    +type: 'asset',
+    +details: AssetFile,
+  |}
+  ;
+
 export type LibraryOptions = {|
   dependencies?: Array<string>,
   platform?: string,
@@ -145,7 +160,7 @@ export type Base64Content = string;
 export type AssetContentsByPath = {[destFilePath: string]: Base64Content};
 
 export type Library = {|
-  files: Array<TransformedFile>,
+  +files: Array<TransformedCodeFile>,
   /* cannot be a Map because it's JSONified later on */
-  assets: AssetContentsByPath,
+  +assets: AssetContentsByPath,
 |};
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/__tests__/optimize-module-test.js b/packages/metro-bundler/src/ModuleGraph/worker/__tests__/optimize-module-test.js
index efec7f3a..6dadf689 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/__tests__/optimize-module-test.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/__tests__/optimize-module-test.js
@@ -36,7 +36,7 @@ describe('optimizing JS modules', () => {
       if (error) {
         throw error;
       }
-      transformResult = JSON.stringify(result);
+      transformResult = JSON.stringify(result.details);
       done();
     });
   });
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/__tests__/transform-module-test.js b/packages/metro-bundler/src/ModuleGraph/worker/__tests__/transform-module-test.js
index 105e84b4..2f5422d1 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/__tests__/transform-module-test.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/__tests__/transform-module-test.js
@@ -45,7 +45,8 @@ describe('transforming JS modules:', () => {
 
   it('passes through file name and code', done => {
     transformModule(sourceCode, options(), (error, result) => {
-      expect(result).toEqual(expect.objectContaining({
+      expect(result.type).toBe('code');
+      expect(result.details).toEqual(expect.objectContaining({
         code: sourceCode,
         file: filename,
       }));
@@ -57,21 +58,24 @@ describe('transforming JS modules:', () => {
     const hasteID = 'TheModule';
     const codeWithHasteID = `/** @providesModule ${hasteID} */`;
     transformModule(codeWithHasteID, options(), (error, result) => {
-      expect(result).toEqual(expect.objectContaining({hasteID}));
+      expect(result.type).toBe('code');
+      expect(result.details).toEqual(expect.objectContaining({hasteID}));
       done();
     });
   });
 
   it('sets `type` to `"module"` by default', done => {
     transformModule(sourceCode, options(), (error, result) => {
-      expect(result).toEqual(expect.objectContaining({type: 'module'}));
+      expect(result.type).toBe('code');
+      expect(result.details).toEqual(expect.objectContaining({type: 'module'}));
       done();
     });
   });
 
   it('sets `type` to `"script"` if the input is a polyfill', done => {
     transformModule(sourceCode, {...options(), polyfill: true}, (error, result) => {
-      expect(result).toEqual(expect.objectContaining({type: 'script'}));
+      expect(result.type).toBe('code');
+      expect(result.details).toEqual(expect.objectContaining({type: 'script'}));
       done();
     });
   });
@@ -105,7 +109,7 @@ describe('transforming JS modules:', () => {
     transformModule(sourceCode, options(), (error, result) => {
       expect(error).toEqual(null);
 
-      const {code, dependencyMapName} = result.transformed.default;
+      const {code, dependencyMapName} = result.details.transformed.default;
       expect(code.replace(/\s+/g, ''))
         .toEqual(
           `__d(function(global,require,module,exports,${
@@ -119,7 +123,7 @@ describe('transforming JS modules:', () => {
     transformModule(sourceCode, {...options(), polyfill: true}, (error, result) => {
       expect(error).toEqual(null);
 
-      const {code} = result.transformed.default;
+      const {code} = result.details.transformed.default;
       expect(code.replace(/\s+/g, ''))
         .toEqual(`(function(global){${transformedCode}})(this);`);
       done();
@@ -128,7 +132,7 @@ describe('transforming JS modules:', () => {
 
   it('creates source maps', done => {
     transformModule(sourceCode, options(), (error, result) => {
-      const {code, map} = result.transformed.default;
+      const {code, map} = result.details.transformed.default;
       const column = code.indexOf('code');
       const consumer = new SourceMapConsumer(map);
       expect(consumer.originalPositionFor({line: 1, column}))
@@ -145,7 +149,7 @@ describe('transforming JS modules:', () => {
     transformer.transform.stub.returns(transformResult(body));
 
     transformModule(code, options(), (error, result) => {
-      expect(result.transformed.default)
+      expect(result.details.transformed.default)
         .toEqual(expect.objectContaining({dependencies: [dep1, dep2]}));
       done();
     });
@@ -160,7 +164,7 @@ describe('transforming JS modules:', () => {
         .returns(transformResult([]));
 
     transformModule(sourceCode, options(variants), (error, result) => {
-      const {dev, prod} = result.transformed;
+      const {dev, prod} = result.details.transformed;
       expect(dev.code.replace(/\s+/g, ''))
         .toEqual(
           `__d(function(global,require,module,exports,${
@@ -179,7 +183,7 @@ describe('transforming JS modules:', () => {
     const json = '{"foo":"bar"}';
 
     transformModule(json, {...options(), filename: 'some.json'}, (error, result) => {
-      const {code} = result.transformed.default;
+      const {code} = result.details.transformed.default;
       expect(code.replace(/\s+/g, ''))
         .toEqual(
           '__d(function(global,require,module,exports){' +
@@ -191,7 +195,7 @@ describe('transforming JS modules:', () => {
 
   it('does not create source maps for JSON files', done => {
     transformModule('{}', {...options(), filename: 'some.json'}, (error, result) => {
-      expect(result.transformed.default)
+      expect(result.details.transformed.default)
         .toEqual(expect.objectContaining({map: null}));
       done();
     });
@@ -209,7 +213,7 @@ describe('transforming JS modules:', () => {
       JSON.stringify(pkg),
       {...options(), filename: 'arbitrary/package.json'},
       (error, result) => {
-        expect(result.package).toEqual(pkg);
+        expect(result.details.package).toEqual(pkg);
         done();
       },
     );
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js b/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js
index 3369f7d1..f80aa426 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js
@@ -18,7 +18,7 @@ const inline = require('../../JSTransformer/worker/inline').plugin;
 const minify = require('../../JSTransformer/worker/minify');
 const sourceMap = require('source-map');
 
-import type {TransformedFile, TransformResult} from '../types.flow';
+import type {TransformedCodeFile, TransformResult} from '../types.flow';
 
 export type OptimizationOptions = {|
   dev: boolean,
@@ -27,9 +27,9 @@ export type OptimizationOptions = {|
 |};
 
 function optimizeModule(
-  data: string | TransformedFile,
+  data: string | TransformedCodeFile,
   optimizationOptions: OptimizationOptions,
-): TransformedFile {
+): TransformedCodeFile {
   if (typeof data === 'string') {
     data = JSON.parse(data);
   }
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js b/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js
index aba4191a..2e7cf2e1 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/transform-module.js
@@ -20,7 +20,8 @@ const {basename} = require('path');
 
 import type {
   Callback,
-  TransformedFile,
+  TransformedCodeFile,
+  TransformedSourceFile,
   Transformer,
   TransformerResult,
   TransformResult,
@@ -41,7 +42,7 @@ const polyfillFactoryParameters = ['global'];
 function transformModule(
   content: Buffer,
   options: TransformOptions,
-  callback: Callback<TransformedFile>,
+  callback: Callback<TransformedSourceFile>,
 ): void {
   if (options.filename.endsWith('.png')) {
     transformAsset(content, options, callback);
@@ -86,12 +87,15 @@ function transformModule(
     const annotations = docblock.parseAsObject(docblock.extract(code));
 
     callback(null, {
-      assetContent: null,
-      code,
-      file: filename,
-      hasteID: annotations.providesModule || null,
-      transformed,
-      type: options.polyfill ? 'script' : 'module',
+      type: 'code',
+      details: {
+        assetContent: null,
+        code,
+        file: filename,
+        hasteID: annotations.providesModule || null,
+        transformed,
+        type: options.polyfill ? 'script' : 'module',
+      },
     });
   });
   return;
@@ -116,7 +120,7 @@ function transformJSON(json, options, callback) {
     .keys(options.variants || defaultVariants)
     .forEach(key => (transformed[key] = moduleData));
 
-  const result: TransformedFile = {
+  const result: TransformedCodeFile = {
     assetContent: null,
     code: json,
     file: filename,
@@ -133,20 +137,19 @@ function transformJSON(json, options, callback) {
       'react-native': value['react-native'],
     };
   }
-  callback(null, result);
+  callback(null, {type: 'code', details: result});
 }
 
 function transformAsset(
   content: Buffer,
   options: TransformOptions,
-  callback: Callback<TransformedFile>,
+  callback: Callback<TransformedSourceFile>,
 ) {
   callback(null, {
-    assetContent: content.toString('base64'),
-    code: '',
-    file: options.filename,
-    hasteID: null,
-    transformed: {},
+    details: {
+      assetContentBase64: content.toString('base64'),
+      filePath: options.filename,
+    },
     type: 'asset',
   });
 }