Support async/await in dart2dart.

pkg/compiler/lib/src/dart2js.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 dart2js.cmdline;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 392 matching lines @@
           "Option '--analyze-all' implies '--analyze-only'.",
           api.Diagnostic.INFO);
     }
     diagnosticHandler.info(
         "Options $optionsImplyCompilation indicate that output is expected, "
         "but compilation is turned off by the option '--analyze-only'.",
         api.Diagnostic.INFO);
   }
   if (analyzeAll) analyzeOnly = true;
   if (!analyzeOnly) {
-    if (enableAsyncAwait) {
+    if (enableAsyncAwait && outputLanguage != OUTPUT_LANGUAGE_DART) {
       helpAndFail("Option '--enable-async' is currently only supported in "
                   "combination with the '--analyze-only' option.");
     }
     if (allowNativeExtensions) {
       helpAndFail("Option '--allow-native-extensions' is only supported in "
                   "combination with the '--analyze-only' option.");
     }
   }
 
   diagnosticHandler.info('Package root is $packageRoot');
@@ skipping 298 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }