Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/mirrors/analyze.dart

 // Copyright (c) 2012, 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 dart2js.source_mirrors.analyze;
 
 import 'dart:async';
 
 import 'source_mirrors.dart';
 import 'dart2js_mirrors.dart' show Dart2JsMirrorSystem;
@@ skipping 20 matching lines @@
     throw new ArgumentError("libraryRoot must end with a /");
   }
   if (packageRoot != null && !packageRoot.path.endsWith("/")) {
     throw new ArgumentError("packageRoot must end with a /");
   }
   options = new List<String>.from(options);
   options.add('--analyze-only');
   options.add('--analyze-signatures-only');
   options.add('--analyze-all');
   options.add('--categories=Client,Server');
+  options.add('--enable-async');
 
   bool compilationFailed = false;
   void internalDiagnosticHandler(Uri uri, int begin, int end,
                                  String message, api.Diagnostic kind) {
     if (kind == api.Diagnostic.ERROR ||
         kind == api.Diagnostic.CRASH) {
       compilationFailed = true;
     }
     diagnosticHandler(uri, begin, end, message, kind);
   }
 
   Compiler compiler = new apiimpl.Compiler(inputProvider,
                                            null,
                                            internalDiagnosticHandler,
                                            libraryRoot, packageRoot,
                                            options,
                                            const {});
   compiler.librariesToAnalyzeWhenRun = libraries;
   return compiler.run(null).then((bool success) {
     if (success && !compilationFailed) {
       return new Dart2JsMirrorSystem(compiler);
     } else {
       throw new StateError('Failed to create mirror system.');
     }
   });
 }