Settings menu item in stock2 doesn't work

The underlying problem is that we lacked a RenderObjectWrapper for the
RenderView, which meant we couldn't handle changing the RenderObject that was
the root of the RenderView. This CL introduces a RenderViewWrapper and uses it
in a new AppContainer widget root. This change allows us to make App a
non-magical Component that is inserted into the AppContainer in the newly
introduced runApp function.

R=ianh@google.com

Review URL: https://codereview.chromium.org/1184823006.

engine/src/flutter/home.dart

@@ -79,5 +79,5 @@ class SkyHome extends App {
 }
 
 void main() {
-  new SkyHome();
+  runApp(new SkyHome());
 }