Control whether Metro tells Babel to lookup .babelrc files

Summary:
This adds support to RN's configuration file to let people turn off Babel's behavior of looking up .babelrc files. Most of the support for this feature is in Metro (https://github.com/facebook/metro-bundler/pull/31).
Closes https://github.com/facebook/react-native/pull/15136

Differential Revision: D5483241

Pulled By: jeanlauliac

fbshipit-source-id: c78096c1574c9f844c9f34aff73e6f97cb0b5e45

local-cli/server/runServer.js

@@ -152,6 +152,7 @@ function getPackagerServer(args, config) {
     assetRegistryPath: ASSET_REGISTRY_PATH,
     blacklistRE: config.getBlacklistRE(),
     cacheVersion: '3',
+    enableBabelRCLookup: config.getEnableBabelRCLookup(),
     extraNodeModules: config.extraNodeModules,
     getPolyfills: config.getPolyfills,
     getTransformOptions: config.getTransformOptions,

local-cli/util/Config.js

@@ -49,6 +49,13 @@ export type ConfigT = {
    */
   getBlacklistRE(): RegExp,
 
+  /**
+   * Specify whether or not to enable Babel's behavior for looking up .babelrc
+   * files. If false, only the .babelrc file (if one exists) in the main project
+   * root is used.
+   */
+  getEnableBabelRCLookup(): boolean,
+
   /**
    * Specify any additional polyfill modules that should be processed
    * before regular module loading.
@@ -164,6 +171,7 @@ const Config = {
     extraNodeModules: Object.create(null),
     getAssetExts: () => [],
     getBlacklistRE: () => blacklist(),
+    getEnableBabelRCLookup: () => true,
     getPlatforms: () => [],
     getPolyfillModuleNames: () => [],
     getProjectRoots: () => {