fix(WebWorker): Fix bug causing multi browser demo to crash

Closes angular#4839

Author: jteplitz

modules/playground/src/web_workers/todo/server_index.dart

@@ -6,8 +6,10 @@ import "package:angular2/src/web_workers/worker/application_common.dart"
 import 'dart:io';
 import "package:angular2/src/core/reflection/reflection_capabilities.dart";
 import "package:angular2/src/core/reflection/reflection.dart";
+import "package:angular2/src/core/dom/html_adapter.dart";
 
 void main() {
+  Html5LibDomAdapter.makeCurrent();
   reflector.reflectionCapabilities = new ReflectionCapabilities();
   HttpServer.bind('127.0.0.1', 1337).then((HttpServer server) {
     var bus = new MultiClientServerMessageBus.fromHttpServer(server);