Pass the maxWorkers config param correctly to Metro

Summary:
@public

The React Native CLI is not passing the default `maxWorkers` param to Metro, making it run in a single thread and making the development process really slow.

This fixes facebook/metro#253

Reviewed By: mjesun

Differential Revision: D9915500

fbshipit-source-id: d15030af582e99fe20535c07e751cfe12e444f2f

Author: rafeca

local-cli/server/server.js

@@ -32,8 +32,8 @@ module.exports = {
   options: [
     {
       command: '--port [number]',
-      default: process.env.RCT_METRO_PORT || 8081,
       parse: (val: string) => Number(val),
+      default: (config: ConfigT) => config.server.port,
     },
     {
       command: '--host [string]',
@@ -42,18 +42,14 @@ module.exports = {
     {
       command: '--projectRoot [string]',
       description: 'Specify the main project root',
-      default: (config: ConfigT) => {
-        return config.projectRoot;
-      },
+      default: (config: ConfigT) => config.projectRoot,
     },
     {
       command: '--watchFolders [list]',
       description:
         'Specify any additional folders to be added to the watch list',
       parse: (val: string) => val.split(','),
-      default: (config: ConfigT) => {
-        return config.watchFolders;
-      },
+      default: (config: ConfigT) => config.watchFolders,
     },
     {
       command: '--assetExts [list]',
@@ -81,18 +77,15 @@ module.exports = {
       description:
         'Specify any npm packages that import dependencies with providesModule',
       parse: (val: string) => val.split(','),
-      default: (config: RNConfig) => {
-        return config.resolver
-          ? config.resolver.providesModuleNodeModules
-          : undefined;
-      },
+      default: (config: ConfigT) => config.resolver.providesModuleNodeModules,
     },
     {
       command: '--max-workers [number]',
       description:
         'Specifies the maximum number of workers the worker-pool ' +
         'will spawn for transforming files. This defaults to the number of the ' +
         'cores available on your machine.',
+      default: (config: ConfigT) => config.maxWorkers,
       parse: (workers: string) => Number(workers),
     },
     {

local-cli/util/Config.js

@@ -76,6 +76,9 @@ const Config = {
       ],
       getPolyfills,
     },
+    server: {
+      port: process.env.RCT_METRO_PORT || 8081,
+    },
     transformer: {
       babelTransformerPath: require.resolve('metro/src/reactNativeTransformer'),
     },