BREAKING: Bump metro-bundler to v0.21.0

Summary:
`metro-bundler` v0.21 contains a rewritten bundling mechanism, with simplified logic and much faster rebuild times, called delta bundler. This release contains a couple of breaking changes:

* Now, when using a custom transformer, the list of additional babel plugins to apply are passed to the `transform()` method. These are used in non-dev mode for optimization purposes (Check 367a5f5db8 (diff-40653f0c822ac59a5af13d5b4ab31d84) to see how to handle them from the transformer).
* Now, when using a custom transformer outputting `rawMappings`, the transformer does not need to call the `compactMappings` method before returning (check d74685fd1d (diff-40653f0c822ac59a5af13d5b4ab31d84) for more info).
* We've removed support for two config parameters: `postProcessModules` and `postProcessBundleSourcemap`.

Reviewed By: davidaurelio

Differential Revision: D6186035

fbshipit-source-id: 242c5c2a954c6b9b6f339d345f888eaa44704579

local-cli/server/runServer.js

@@ -17,12 +17,15 @@ require('../../setupBabel')();
  * found when Flow v0.54 was deployed. To see the error delete this comment and
  * run Flow. */
 const ReactPackager = require('metro-bundler');
+
+const HmrServer = require('metro-bundler/src/HmrServer');
+
 /* $FlowFixMe(>=0.54.0 site=react_native_oss) This comment suppresses an error
  * found when Flow v0.54 was deployed. To see the error delete this comment and
  * run Flow. */
 const Terminal = require('metro-bundler/src/lib/Terminal');
 
-const attachHMRServer = require('./util/attachHMRServer');
+const attachWebsocketServer = require('./util/attachWebsocketServer');
 /* $FlowFixMe(>=0.54.0 site=react_native_oss) This comment suppresses an error
  * found when Flow v0.54 was deployed. To see the error delete this comment and
  * run Flow. */
@@ -131,10 +134,10 @@ function runServer(
     : http.createServer(app);
 
   serverInstance.listen(args.port, args.host, 511, function() {
-    attachHMRServer({
+    attachWebsocketServer({
       httpServer: serverInstance,
       path: '/hot',
-      packagerServer,
+      websocketServer: new HmrServer(packagerServer, reporter),
     });
 
     wsProxy = webSocketProxy.attachToServer(serverInstance, '/debugger-proxy');
@@ -200,7 +203,6 @@ function getPackagerServer(args, config, reporter) {
     sourceExts: defaultSourceExts.concat(args.sourceExts),
     transformModulePath: transformModulePath,
     transformCache: TransformCaching.useTempDir(),
-    useDeltaBundler: false,
     verbose: args.verbose,
     watch: !args.nonPersistent,
     workerPath: config.getWorkerPath(),