Make Completers asynchronous.

runtime/lib/isolate_patch.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.
 
 class _CloseToken {
   /// This token is sent from [IsolateSink]s to [IsolateStream]s to ask them to
   /// close themselves.
   const _CloseToken();
 }
 
@@ skipping 118 matching lines @@
   void send(var message, [SendPort replyTo = null]) {
     this._sendNow(message, replyTo);
   }
 
   void _sendNow(var message, SendPort replyTo) {
     int replyId = (replyTo == null) ? 0 : replyTo._id;
     _sendInternal(_id, replyId, message);
   }
 
   Future call(var message) {
-    final completer = new Completer();
+    final completer = new Completer.sync();
     final port = new _ReceivePortImpl();
     send(message, port.toSendPort());
     port.receive((value, ignoreReplyTo) {
       port.close();
       if (value is Exception) {
         completer.completeError(value);
       } else {
         completer.complete(value);
       }
     });
@@ skipping 36 matching lines @@
     }
     return _portInternal;
   }
 
   /* patch */ static spawnFunction(void topLevelFunction(),
       [bool unhandledExceptionCallback(IsolateUnhandledException e)])
       native "isolate_spawnFunction";
 
   /* patch */ static spawnUri(String uri) native "isolate_spawnUri";
 }