SkyDemo should launch demos in their own windows

sky/sdk/lib/mojo/activity.dart

Index: sky/sdk/lib/mojo/activity.dart
diff --git a/sky/sdk/lib/mojo/activity.dart b/sky/sdk/lib/mojo/activity.dart
index 5ddfc7db28af192d147ff4714a5e6814f3d19d0a..7abd84d06f95bd7bdcc1537dd14ce49b35411a36 100644
--- a/sky/sdk/lib/mojo/activity.dart
+++ b/sky/sdk/lib/mojo/activity.dart
@@ -5,8 +5,22 @@
 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);
+}