fix(WebWorker): Fix Todo Server demo and add test to ensure the demo can bootstrap.

Closes #3970

modules/angular2/test/web_workers/shared/mock_event_emitter.dart

@@ -5,17 +5,16 @@ import 'dart:async';
 import "package:angular2/src/core/facade/async.dart";
 
 class MockEventEmitter extends EventEmitter {
-  List<Function> _nextFns = new List();
+  final controller = new StreamController.broadcast(sync: true);
 
   @override
   StreamSubscription listen(void onData(dynamic line),
       {void onError(Error error), void onDone(), bool cancelOnError}) {
-    _nextFns.add(onData);
-    return null;
+    return controller.stream.listen(onData, onError: onError, onDone: onDone, cancelOnError: cancelOnError);
   }
 
   @override
   void add(value) {
-    _nextFns.forEach((fn) => fn(value));
+    controller.add(value);
   }
 }

modules/angular2/test/web_workers/shared/mock_event_emitter.ts

@@ -9,10 +9,14 @@ export class MockEventEmitter extends EventEmitter {
 
   observer(generator: any): Rx.IDisposable {
     this._nextFns.push(generator.next);
-    return null;
+    return new MockDisposable();
   }
 
   next(value: any) {
     ListWrapper.forEach(this._nextFns, (fn) => { fn(value); });
   }
 }
+
+class MockDisposable implements Rx.IDisposable {
+  dispose(): void {}
+}