Support asynchronuous spawnDomFunction.

tools/dom/src/native_DOMImplementation.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.
 
 part of html;
 
 class _Utils {
   static double dateTimeToDouble(DateTime dateTime) =>
       dateTime.millisecondsSinceEpoch.toDouble();
   static DateTime doubleToDateTime(double dateTime) {
@@ skipping 30 matching lines @@
 
   static Map createMap() => {};
 
   static makeUnimplementedError(String fileName, int lineNo) {
     return new UnsupportedError('[info: $fileName:$lineNo]');
   }
 
   static window() native "Utils_window";
   static print(String message) native "Utils_print";
   static forwardingPrint(String message) native "Utils_forwardingPrint";
-  static SendPort spawnDomFunctionImpl(Function topLevelFunction) native "Utils_spawnDomFunction";
+  static void spawnDomFunction(Function topLevelFunction, int replyTo) native "Utils_spawnDomFunction";
   static int _getNewIsolateId() native "Utils_getNewIsolateId";
   static bool shadowRootSupported(Document document) native "Utils_shadowRootSupported";
 }
 
 class _NPObject extends NativeFieldWrapperClass1 {
   _NPObject.internal();
   static _NPObject retrieve(String key) native "NPObject_retrieve";
   property(String propertyName) native "NPObject_property";
   invoke(String methodName, [List args = null]) native "NPObject_invoke";
 }
@@ skipping 60 matching lines @@
 
 get _printClosure => (s) {
   try {
     window.console.log(s);
   } catch (_) {
     _Utils.print(s);
   }
 };
 
 final _forwardingPrintClosure = _Utils.forwardingPrint;