Simplify ReceivePortSync.receive implementation.

lib/html/src/Isolates.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.
 
 _serialize(var message) {
   return new _JsSerializer().traverse(message);
 }
 
 class _JsSerializer extends _Serializer {
 
@@ skipping 139 matching lines @@
       _cachedIsolateId = _getNewIsolateId();      
     }
     return _cachedIsolateId;
   }
 
   static String _getListenerName(isolateId, portId) =>
       'dart-port-$isolateId-$portId'; 
   String get _listenerName() => _getListenerName(_isolateId, _portId);
 
   void receive(callback(var message)) {
-    // Clear old listener.
-    if (_callback != null) {
-      window.on[_listenerName].remove(_listener);
+    _callback = callback;
+    if (_listener === null) {
+      _listener = (TextEvent e) {
+        var data = JSON.parse(e.data);
+        var replyTo = data[0];
+        var message = _deserialize(data[1]);
+        var result = _callback(message);
+        _dispatchEvent(replyTo, _serialize(result));
+      };
+      window.on[_listenerName].add(_listener);
     }
-
-    _callback = callback;
-
-    // Install new listener.
-    var sendport = toSendPort();
-    _listener = (TextEvent e) {
-      var data = JSON.parse(e.data);
-      var replyTo = data[0];
-      var message = _deserialize(data[1]);
-      var result = sendport.callSync(message);
-      _dispatchEvent(replyTo, _serialize(result));
-    };
-    window.on[_listenerName].add(_listener);
   }
 
   void close() {
     _portMap.remove(_portId);
-    window.on[_listenerName].remove(_listener);
+    if (_listener !== null) window.on[_listenerName].remove(_listener);
   }
 
   SendPortSync toSendPort() {
     return new _LocalSendPortSync._internal(this);
   }
 
   static SendPortSync _lookup(int isolateId, int portId) {
     if (isolateId == _isolateId) {
       return _portMap[portId].toSendPort();
     } else {
       return new _RemoteSendPortSync(isolateId, portId);
     }
   }
 }
 
 void _dispatchEvent(String receiver, var message) {
   var event = document.$dom_createEvent('TextEvent');
   event.initTextEvent(receiver, false, false, window, JSON.stringify(message));
   window.$dom_dispatchEvent(event);
 }