[Effen] Try to catch more misuse of fn by asserting that UINode nodes are only built when we're upd…

sky/sdk/lib/framework/fn.dart

Index: sky/sdk/lib/framework/fn.dart
diff --git a/sky/sdk/lib/framework/fn.dart b/sky/sdk/lib/framework/fn.dart
index 9f51a17da881bb1d1bc64ca004f17b18cafe46d3..dc54b76f1c61355e05699162c2d103a3cb1a9436 100644
--- a/sky/sdk/lib/framework/fn.dart
+++ b/sky/sdk/lib/framework/fn.dart
@@ -32,6 +32,7 @@ abstract class UINode {
 
   UINode({ Object key }) {
     _key = key == null ? "$runtimeType" : "$runtimeType-$key";
+    assert(this is App || _inRenderDirtyComponents); // you should not build the UI tree ahead of time, build it only during build()
   }
 
   // Subclasses which implements Nodes that become stateful may return true