Add an assert that we don't schedule a render during render

sky/framework/fn.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 library fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 import 'reflect.dart' as reflect;
@@ skipping 485 matching lines @@
 
     sky.HTMLAnchorElement skyAnchor = _root as sky.HTMLAnchorElement;
     if (href != oldAnchor.href) {
       skyAnchor.href = href;
     }
   }
 }
 
 List<Component> _dirtyComponents = new List<Component>();
 bool _renderScheduled = false;
+bool _inRenderDirtyComponents = false;
 
 void _renderDirtyComponents() {
-  Stopwatch sw = new Stopwatch()..start();
+  try {
+    _inRenderDirtyComponents = true;
+    Stopwatch sw = new Stopwatch()..start();
 
-  _dirtyComponents.sort((a, b) => a._order - b._order);
-  for (var comp in _dirtyComponents) {
-    comp._renderIfDirty();
+    _dirtyComponents.sort((a, b) => a._order - b._order);
+    for (var comp in _dirtyComponents) {
+      comp._renderIfDirty();
+    }
+
+    _dirtyComponents.clear();
+    _renderScheduled = false;
+
+    sw.stop();
+    if (_shouldLogRenderDuration)
+      print("Render took ${sw.elapsedMicroseconds} microseconds");
+  } finally {
+    _inRenderDirtyComponents = false;
   }
-
-  _dirtyComponents.clear();
-  _renderScheduled = false;
-
-  sw.stop();
-  if (_shouldLogRenderDuration)
-    print("Render took ${sw.elapsedMicroseconds} microseconds");
 }
 
 void _scheduleComponentForRender(Component c) {
+  assert(!_inRenderDirtyComponents);
+  if (_inRenderDirtyComponents)
+    return;
+
   _dirtyComponents.add(c);
 
   if (!_renderScheduled) {
     _renderScheduled = true;
     new Future.microtask(_renderDirtyComponents);
   }
 }
 
 abstract class Component extends Node {
   bool _dirty = true; // components begin dirty because they haven't rendered.
@@ skipping 119 matching lines @@
 
       _sync(null, _host, null);
       assert(_root is sky.Node);
 
       sw.stop();
       if (_shouldLogRenderDuration)
         print("Initial render: ${sw.elapsedMicroseconds} microseconds");
     });
   }
 }