SkyDemo should launch demos in their own windows

sky/sdk/lib/mojo/activity.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:mojom/intents/intents.mojom.dart';
 import 'package:sky/mojo/shell.dart' as shell;
 
+const int NEW_DOCUMENT = 0x00080000;
+const int NEW_TASK = 0x10000000;
+const int MULTIPLE_TASK = 0x08000000;
+
+ActivityManagerProxy _initActivityManager() {
+  ActivityManagerProxy activityManager = new ActivityManagerProxy.unbound();
+  shell.requestService('mojo:sky_viewer', activityManager);
+  return activityManager;
+}
+
+final ActivityManagerProxy _activityManager = _initActivityManager();
+
 void finishCurrentActivity() {
-    ActivityManagerProxy activityManager = new ActivityManagerProxy.unbound();
-    shell.requestService('mojo:sky_viewer', activityManager);
-    activityManager.ptr.finishCurrentActivity();
-}
+  _activityManager.ptr.finishCurrentActivity();
+}
+
+void startActivity(Intent intent) {
+  _activityManager.ptr.startActivity(intent);
+}