Make it possible to test that the stock app doesn't crash on startup and paints the basic scaffold …

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 7785088e6970708ec7fb55c78fe63a25710012fd..701821c24f4a2017dd5cf0bd4c1419e775fe2718 100644
--- a/sky/sdk/lib/framework/app.dart
+++ b/sky/sdk/lib/framework/app.dart
@@ -16,7 +16,7 @@ class PointerState {
 
 class AppView {
 
-  AppView([RenderBox root = null]) {
+  AppView({ RenderBox root: null, RenderView renderViewOverride }) {
     assert(_app == null);
     _app = this;
 
@@ -25,10 +25,15 @@ class AppView {
     scheduler.init();
     scheduler.addPersistentFrameCallback(_beginFrame);
 
-    _renderView = new RenderView(child: root);
-    _renderView.attach();
-    _renderView.rootConstraints = _viewConstraints;
-    _renderView.scheduleInitialLayout();
+    if (renderViewOverride == null) {
+      _renderView = new RenderView(child: root);
+      _renderView.attach();
+      _renderView.rootConstraints = _viewConstraints;
+      _renderView.scheduleInitialLayout();
+    } else {
+      _renderView = renderViewOverride;
+    }
+    assert(_renderView != null);
 
     assert(_app == this);
   }