Add flag to pub serve to disable dart2js.

sdk/lib/_internal/pub/lib/src/command/serve.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.command.serve;
 
 import 'dart:async';
 
+import '../barback/dart_forwarding_transformer.dart';
+import '../barback/dart2js_transformer.dart';
+import '../barback/pub_package_provider.dart';
 import '../barback.dart' as barback;
-import '../barback/pub_package_provider.dart';
 import '../command.dart';
 import '../entrypoint.dart';
 import '../exit_codes.dart' as exit_codes;
 import '../io.dart';
 import '../log.dart' as log;
 import '../utils.dart';
 
 final _green = getSpecial('\u001b[32m');
 final _red = getSpecial('\u001b[31m');
 final _none = getSpecial('\u001b[0m');
 final _arrow = getSpecial('\u2192', '=>');
 
 /// Handles the `serve` pub command.
 class ServeCommand extends PubCommand {
   String get description => "Run a local web development server.";
   String get usage => 'pub serve';
 
   PubPackageProvider _provider;
 
   String get hostname => commandOptions['hostname'];
 
+  /// `true` if Dart entrypoints should be compiled to JavaScript.
+  bool get useDart2JS => commandOptions['dart2js'];
+
   ServeCommand() {
     commandParser.addOption('port', defaultsTo: '8080',
         help: 'The port to listen on.');
 
     // A hidden option for the tests to work around a bug in some of the OS X
     // bots where "localhost" very rarely resolves to the IPv4 loopback address
     // instead of IPv6 (or vice versa). The tests will always set this to
     // 127.0.0.1.
     commandParser.addOption('hostname',
                             defaultsTo: 'localhost',
                             hide: true);
+
+    commandParser.addFlag('dart2js', defaultsTo: true,
+        help: 'Compile Dart to JavaScript.');
   }
 
   Future onRun() {
     var port;
     try {
       port = int.parse(commandOptions['port']);
     } on FormatException catch (_) {
       log.error('Could not parse port "${commandOptions['port']}"');
       this.printUsage();
       return flushThenExit(exit_codes.USAGE);
     }
 
-    return ensureLockFileIsUpToDate()
-        .then((_) => entrypoint.loadPackageGraph())
-        .then((graph) => barback.createServer(hostname, port, graph))
-        .then((server) {
+    return ensureLockFileIsUpToDate().then((_) {
+      return entrypoint.loadPackageGraph();
+    }).then((graph) {
+      // TODO(rnystrom): Add support for dart2dart transformer here.
+      var builtInTransformers = null;
+      if (useDart2JS) {
+        builtInTransformers = [
+          new Dart2JSTransformer(graph),
+          new DartForwardingTransformer()
+        ];
+      }
+
+      return barback.createServer(hostname, port, graph,
+          builtInTransformers: builtInTransformers);
+    }).then((server) {
       /// This completer is used to keep pub running (by not completing) and
       /// to pipe fatal errors to pub's top-level error-handling machinery.
       var completer = new Completer();
 
       server.barback.errors.listen((error) {
         log.error("${_red}Build error:\n$error$_none");
       });
 
       server.barback.results.listen((result) {
         if (result.succeeded) {
@@ skipping 40 matching lines @@
       // needed.
       if (!entrypoint.isLockFileUpToDate()) {
         log.message("Dependencies have changed, installing...");
         return entrypoint.installDependencies().then((_) {
           log.message("Dependencies installed!");
         });
       }
     });
   }
 }