Abstract out the AppView logic in fn so that it can also be used in non-pure-fn apps.

sky/sdk/lib/framework/app.dart

Index: sky/sdk/lib/framework/app.dart
diff --git a/sky/sdk/lib/framework/app.dart b/sky/sdk/lib/framework/app.dart
index 0e3787b7e1cc58befe022213f92cca712912cc06..7785088e6970708ec7fb55c78fe63a25710012fd 100644
--- a/sky/sdk/lib/framework/app.dart
+++ b/sky/sdk/lib/framework/app.dart
@@ -16,7 +16,10 @@ class PointerState {
 
 class AppView {
 
-  AppView(RenderBox root) {
+  AppView([RenderBox root = null]) {
+    assert(_app == null);
+    _app = this;
+
     sky.view.setEventCallback(_handleEvent);
     sky.view.setMetricsChangedCallback(_handleMetricsChanged);
     scheduler.init();
@@ -26,8 +29,12 @@ class AppView {
     _renderView.attach();
     _renderView.rootConstraints = _viewConstraints;
     _renderView.scheduleInitialLayout();
+
+    assert(_app == this);
   }
 
+  static AppView _app; // used to enforce that we're a singleton
+
   RenderView _renderView;
 
   ViewConstraints get _viewConstraints =>