Support async/await in dart2dart.

pkg/compiler/lib/src/apiimpl.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 leg_apiimpl;
 
 import 'dart:async';
 
 import '../compiler.dart' as api;
 import 'dart2jslib.dart' as leg;
@@ skipping 88 matching lines @@
     if (!libraryRoot.path.endsWith("/")) {
       throw new ArgumentError("[libraryRoot] must end with a /.");
     }
     if (packageRoot == null) {
       throw new ArgumentError("[packageRoot] is null.");
     }
     if (!packageRoot.path.endsWith("/")) {
       throw new ArgumentError("[packageRoot] must end with a /.");
     }
     if (!analyzeOnly) {
-      if (enableAsyncAwait) {
+      if (enableAsyncAwait && emitJavaScript) {
         throw new ArgumentError(
             "--enable-async is currently only supported with --analyze-only");
       }
       if (allowNativeExtensions) {
         throw new ArgumentError(
             "--allow-native-extensions is only supported in combination with "
             "--analyze-only");
       }
     }
   }
@@ skipping 285 matching lines @@
     print('$message: ${tryToString(exception)}');
     print(tryToString(stackTrace));
   }
 
   fromEnvironment(String name) => environment[name];
 
   LibraryInfo lookupLibraryInfo(String libraryName) {
     return library_info.LIBRARIES[libraryName];
   }
 }