diff --git a/packages/metro-bundler/src/Config.js b/packages/metro-bundler/src/Config.js
index ca98aeaa..f892da9a 100644
--- a/packages/metro-bundler/src/Config.js
+++ b/packages/metro-bundler/src/Config.js
@@ -27,6 +27,8 @@ import type {TransformVariants} from './ModuleGraph/types.flow';
 import type {HasteImpl} from './node-haste/Module';
 
 export type ConfigT = {
+  assetRegistryPath: string,
+
   extraNodeModules: {[id: string]: string},
   /**
    * Specify any additional asset file extensions to be used by the packager.
@@ -140,6 +142,7 @@ export type ConfigT = {
 };
 
 const DEFAULT = ({
+  assetRegistryPath: 'missing-asset-registry-path',
   extraNodeModules: {},
   assetTransforms: false,
   getAssetExts: () => [],
diff --git a/packages/metro-bundler/src/index.js b/packages/metro-bundler/src/index.js
index 4d6bc54e..0e1e99d1 100644
--- a/packages/metro-bundler/src/index.js
+++ b/packages/metro-bundler/src/index.js
@@ -74,7 +74,7 @@ async function asyncRealpath(path): Promise<string> {
 async function runMetro({
   config,
   maxWorkers = 1,
-  projectRoots,
+  projectRoots = [],
   watch = false,
 }: PrivateMetroOptions) {
   const normalizedConfig = config ? normalizeConfig(config) : DEFAULT_CONFIG;
@@ -97,7 +97,7 @@ async function runMetro({
 
   const serverOptions: ServerOptions = {
     assetExts: defaultAssetExts.concat(assetExts),
-    assetRegistryPath: `invalid`,
+    assetRegistryPath: normalizedConfig.assetRegistryPath,
     blacklistRE: normalizedConfig.getBlacklistRE(),
     extraNodeModules: normalizedConfig.extraNodeModules,
     getPolyfills: normalizedConfig.getPolyfills,
@@ -237,12 +237,13 @@ exports.runBuild = async (options: RunBuildOptions) => {
   const metroBundle = await MetroBundler.build(metroServer, requestOptions);
 
   const outputOptions: OutputOptions = {
-    bundleOutput: options.out,
+    bundleOutput: options.out.replace(/(\.js)?$/, '.js'),
     dev: options.dev,
     platform: options.platform || `web`,
   };
 
   await MetroBundler.save(metroBundle, outputOptions, console.log);
+  await metroServer.end();
 
   return {metroServer, metroBundle};
 };