Back button should kill interactive flex app

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
new file mode 100644
index 0000000000000000000000000000000000000000..5ddfc7db28af192d147ff4714a5e6814f3d19d0a
--- /dev/null
+++ b/sky/sdk/lib/mojo/activity.dart
@@ -0,0 +1,12 @@
+// 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;
+
+void finishCurrentActivity() {
+    ActivityManagerProxy activityManager = new ActivityManagerProxy.unbound();
+    shell.requestService('mojo:sky_viewer', activityManager);
+    activityManager.ptr.finishCurrentActivity();
+}