diff --git a/local-cli/bundle/output/unbundle/as-assets.js b/local-cli/bundle/output/unbundle/as-assets.js
index 7a4378c8a7118e..da9e30af50b572 100644
--- a/local-cli/bundle/output/unbundle/as-assets.js
+++ b/local-cli/bundle/output/unbundle/as-assets.js
@@ -12,6 +12,7 @@ const mkdirp = require('mkdirp');
 const path = require('path');
 const Promise = require('promise');
 
+const buildSourceMapWithMetaData = require('./build-unbundle-sourcemap-with-metadata');
 const writeFile = require('../writeFile');
 const writeSourceMap = require('./write-sourcemap');
 const MAGIC_UNBUNDLE_NUMBER = require('./magic-number');
@@ -29,12 +30,11 @@ function saveAsAssets(bundle, options, log) {
   const {
     'bundle-output': bundleOutput,
     'bundle-encoding': encoding,
-    dev,
     'sourcemap-output': sourcemapOutput,
   } = options;
 
   log('start');
-  const {startupCode, modules} = bundle.getUnbundle({minify: !dev});
+  const {startupCode, startupModules, modules} = bundle.getUnbundle();
   log('finish');
 
   log('Writing bundle output to:', bundleOutput);
@@ -49,7 +49,13 @@ function saveAsAssets(bundle, options, log) {
     );
   writeUnbundle.then(() => log('Done writing unbundle output'));
 
-  return Promise.all([writeUnbundle, writeSourceMap(sourcemapOutput, '', log)]);
+  const sourceMap =
+    buildSourceMapWithMetaData({startupModules, modules});
+
+  return Promise.all([
+    writeUnbundle,
+    writeSourceMap(sourcemapOutput, JSON.stringify(sourceMap), log)
+  ]);
 }
 
 function createDir(dirName) {
diff --git a/local-cli/bundle/output/unbundle/build-unbundle-sourcemap-with-metadata.js b/local-cli/bundle/output/unbundle/build-unbundle-sourcemap-with-metadata.js
new file mode 100644
index 00000000000000..9c18c1ef963ea2
--- /dev/null
+++ b/local-cli/bundle/output/unbundle/build-unbundle-sourcemap-with-metadata.js
@@ -0,0 +1,22 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+const {combineSourceMaps} = require('./util');
+
+module.exports = ({startupModules, modules}) => {
+  const startupModule = {
+    code: startupModules.map(m => m.code).join('\n'),
+    map: combineSourceMaps({modules: startupModules}),
+  };
+  return combineSourceMaps({
+    modules: [startupModule].concat(modules),
+    withCustomOffsets: true,
+  });
+};
diff --git a/local-cli/bundle/output/unbundle/util.js b/local-cli/bundle/output/unbundle/util.js
new file mode 100644
index 00000000000000..9c2a14c267fc0c
--- /dev/null
+++ b/local-cli/bundle/output/unbundle/util.js
@@ -0,0 +1,60 @@
+/**
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+'use strict';
+
+const newline = /\r\n?|\n|\u2028|\u2029/g;
+const countLines =
+  string => (string.match(newline) || []).length + 1; // fastest implementation
+
+function lineToLineSourceMap(source, filename) {
+  // The first line mapping in our package is 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;';
+
+  return {
+    version: 3,
+    sources: [filename],
+    mappings: firstLine + Array(countLines(source)).join(line),
+  };
+}
+
+const wrapperEnd = wrappedCode => wrappedCode.indexOf('{') + 1;
+
+const Section = (line, column, map) => ({map, offset: {line, column}});
+
+function combineSourceMaps({modules, withCustomOffsets}) {
+  let offsets;
+  const sections = [];
+  const sourceMap = {
+    version: 3,
+    sections,
+  };
+
+  if (withCustomOffsets) {
+    offsets = sourceMap.x_facebook_offsets = [];
+  }
+
+  let line = 0;
+  modules.forEach(({code, id, map, name}) => {
+    const hasOffset = withCustomOffsets && id != null;
+    const column = hasOffset ? wrapperEnd(code) : 0;
+    sections.push(Section(line, column, map || lineToLineSourceMap(code, name)));
+    if (hasOffset) {
+      offsets[id] = line;
+    }
+    line += countLines(code);
+  });
+
+  return sourceMap;
+}
+
+module.exports = {countLines, lineToLineSourceMap, combineSourceMaps};
diff --git a/packager/react-packager/src/Bundler/Bundle.js b/packager/react-packager/src/Bundler/Bundle.js
index af265ebbc5b658..febac2dba33237 100644
--- a/packager/react-packager/src/Bundler/Bundle.js
+++ b/packager/react-packager/src/Bundler/Bundle.js
@@ -115,9 +115,7 @@ class Bundle extends BundleBase {
 
     return {
       startupCode,
-      modules: modules.map(({name, code, polyfill}) =>
-        ({name, code, polyfill})
-      ),
+      startupModules: allModules,
       modules,
     };
   }