[Effen] add an assert that you don't try to re-add a defunct node

R=abarth@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/1149533002

sdk/lib/framework/fn.dart

@@ -109,6 +109,7 @@ abstract class UINode {
       return oldNode;
     }
 
+    assert(!node._defunct);
     node._parent = this;
 
     if (oldNode == null) {