Prevent already scheduled-for-render fn components which become removed from rendering

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 618 matching lines @@
     }
     _root = _rendered._root;
     assert(_rendered._root is sky.Node);
 
     if (mounting) {
       didMount();
     }
   }
 
   void _renderIfDirty() {
+    if (_removed)
+      return;
+
     assert(_rendered != null);
-    assert(!_removed);
 
     var rendered = _rendered;
     while (rendered is Component) {
       rendered = rendered._rendered;
     }
+
+    assert(rendered._root != null);
     sky.Node root = rendered._root;
 
     _renderInternal(root.parentNode, root.nextSibling);
   }
 
   void setState(Function fn()) {
     assert(_rendered != null || _removed); // cannot setState before mounting.
     _stateful = true;
     fn();
     if (!_removed && _currentlyRendering != this) {
@@ skipping 16 matching lines @@
 
       _sync(null, _host, null);
       assert(_root is sky.Node);
 
       sw.stop();
       if (_shouldLogRenderDuration)
         print("Initial render: ${sw.elapsedMicroseconds} microseconds");
     });
   }
 }