diff --git a/packages/metro-bundler/src/ModuleGraph/ModuleGraph.js b/packages/metro-bundler/src/ModuleGraph/ModuleGraph.js
index 9d67a024..cce7ff8a 100644
--- a/packages/metro-bundler/src/ModuleGraph/ModuleGraph.js
+++ b/packages/metro-bundler/src/ModuleGraph/ModuleGraph.js
@@ -31,8 +31,8 @@ type BuildFn = (
 ) => void;
 
 type BuildOptions = {|
-  optimize?: boolean,
-  platform?: string,
+  optimize: boolean,
+  platform: string,
 |};
 
 exports.createBuildSetup = (
@@ -96,6 +96,7 @@ function* concat<T>(...iterables: Array<Iterable<T>>): Iterable<T> {
 
 function prelude(optimize) {
   return virtualModule(
-    `var __DEV__=${String(!optimize)},__BUNDLE_START_TIME__=Date.now();`
+    `var __DEV__=${String(!optimize)},` +
+    '__BUNDLE_START_TIME__=global.nativePerformanceNow?global.nativePerformanceNow():Date.now();'
   );
 }
diff --git a/packages/metro-bundler/src/ModuleGraph/__tests__/ModuleGraph-test.js b/packages/metro-bundler/src/ModuleGraph/__tests__/ModuleGraph-test.js
index 4a3844c9..d9a9108a 100644
--- a/packages/metro-bundler/src/ModuleGraph/__tests__/ModuleGraph-test.js
+++ b/packages/metro-bundler/src/ModuleGraph/__tests__/ModuleGraph-test.js
@@ -28,7 +28,8 @@ describe('build setup', () => {
       expect(prelude).toEqual({
         dependencies: [],
         file: {
-          code: 'var __DEV__=true,__BUNDLE_START_TIME__=Date.now();',
+          code: 'var __DEV__=true,__BUNDLE_START_TIME__=' +
+            'global.nativePerformanceNow?global.nativePerformanceNow():Date.now();',
           path: '',
           type: 'script',
         },
@@ -41,7 +42,8 @@ describe('build setup', () => {
     buildSetup(noEntryPoints, {optimize: true}, (error, result) => {
       const [prelude] = result.modules;
       expect(prelude.file.code)
-        .toEqual('var __DEV__=false,__BUNDLE_START_TIME__=Date.now();');
+        .toEqual('var __DEV__=false,__BUNDLE_START_TIME__=' +
+            'global.nativePerformanceNow?global.nativePerformanceNow():Date.now();');
       done();
     });
   });
diff --git a/packages/metro-bundler/src/ModuleGraph/worker.js b/packages/metro-bundler/src/ModuleGraph/worker.js
index 96c00f6e..6a1a41f3 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker.js
@@ -10,15 +10,19 @@
  */
 'use strict';
 
-const asyncify = require('async/asyncify');
+const asyncify: Asyncify = require('async/asyncify');
 const optimizeModule = require('./worker/optimize-module');
 const transformModule = require('./worker/transform-module');
 const wrapWorkerFn = require('./worker/wrap-worker-fn');
 
+import type {Callback} from './types.flow';
 import type {OptimizationOptions} from './worker/optimize-module';
 import type {TransformOptions} from './worker/transform-module';
 import type {WorkerFnWithIO} from './worker/wrap-worker-fn';
 
+type Asyncify = <A, B, C>((A, B) => C) => (A, B, Callback<C>) => void;
+
+
 exports.optimizeModule =
   (wrapWorkerFn(asyncify(optimizeModule)): WorkerFnWithIO<OptimizationOptions>);
 exports.transformModule =
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 6dadf689..37e58472 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
@@ -36,23 +36,24 @@ describe('optimizing JS modules', () => {
       if (error) {
         throw error;
       }
-      transformResult = JSON.stringify(result.details);
+      transformResult = JSON.stringify({type: 'code', details: result.details});
       done();
     });
   });
 
   it('copies everything from the transformed file, except for transform results', () => {
     const result = optimizeModule(transformResult, optimizationOptions);
-    const expected = JSON.parse(transformResult);
+    const expected = JSON.parse(transformResult).details;
     delete expected.transformed;
-    expect(result).toEqual(objectContaining(expected));
+    expect(result.type).toBe('code');
+    expect(result.details).toEqual(objectContaining(expected));
   });
 
   describe('code optimization', () => {
     let dependencyMapName, injectedVars, optimized, requireName;
     beforeAll(() => {
       const result = optimizeModule(transformResult, optimizationOptions);
-      optimized = result.transformed.default;
+      optimized = result.details.transformed.default;
       injectedVars = optimized.code.match(/function\(([^)]*)/)[1].split(',');
       [, requireName,,, dependencyMapName] = injectedVars;
     });
@@ -79,10 +80,16 @@ describe('optimizing JS modules', () => {
       const result = optimizeModule(
         transformResult,
         {...optimizationOptions, isPolyfill: true},
-      );
+      ).details;
       expect(result.transformed.default.dependencies).toEqual([]);
     });
   });
+
+  it('passes through non-code data unmodified', () => {
+    const data = {type: 'asset', details: {arbitrary: 'data'}};
+    expect(optimizeModule(JSON.stringify(data), {dev: true, platform: ''}))
+      .toEqual(data);
+  });
 });
 
 function findLast(code, needle) {
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js b/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js
index f80aa426..89885163 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 {TransformedCodeFile, TransformResult} from '../types.flow';
+import type {TransformedSourceFile, TransformResult} from '../types.flow';
 
 export type OptimizationOptions = {|
   dev: boolean,
@@ -27,21 +27,25 @@ export type OptimizationOptions = {|
 |};
 
 function optimizeModule(
-  data: string | TransformedCodeFile,
+  content: Buffer,
   optimizationOptions: OptimizationOptions,
-): TransformedCodeFile {
-  if (typeof data === 'string') {
-    data = JSON.parse(data);
+): TransformedSourceFile {
+  const data: TransformedSourceFile = JSON.parse(content.toString('utf8'));
+
+  if (data.type !== 'code') {
+    return data;
   }
-  const {code, file, transformed} = data;
-  const result = {...data, transformed: {}};
+
+  const {details} = data;
+  const {code, file, transformed} = details;
+  const result = {...details, transformed: {}};
 
   //$FlowIssue #14545724
   Object.entries(transformed).forEach(([k, t: TransformResult]: [*, TransformResult]) => {
     result.transformed[k] = optimize(t, file, code, optimizationOptions);
   });
 
-  return result;
+  return {type: 'code', details: result};
 }
 
 function optimize(transformed, file, originalCode, options): TransformResult {