fix(WebWorker): Don't send messages when the buffer is empty

Closes #4138

modules/angular2/src/web_workers/shared/generic_message_bus.dart

@@ -46,8 +46,10 @@ abstract class GenericMessageBusSink implements MessageBusSink {
   void attachToZone(NgZone zone) {
     _zone = zone;
     _zone.overrideOnEventDone(() {
-      sendMessages(_messageBuffer);
-      _messageBuffer.clear();
+      if (_messageBuffer.length > 0) {
+        sendMessages(_messageBuffer);
+        _messageBuffer.clear();
+      }
     }, false);
   }
 

modules/angular2/src/web_workers/shared/post_message_bus.ts

@@ -73,9 +73,10 @@ export class PostMessageBusSink implements MessageBusSink {
   }
 
   private _handleOnEventDone() {
-    // TODO: Send all buffered messages in one postMessage call
-    this._sendMessages(this._messageBuffer);
-    this._messageBuffer = [];
+    if (this._messageBuffer.length > 0) {
+      this._sendMessages(this._messageBuffer);
+      this._messageBuffer = [];
+    }
   }
 
   private _sendMessages(messages: Array<Object>) { this._postMessageTarget.postMessage(messages); }