Example showing alternate async measurement solution

client/html/src/CssClassSet.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO - figure out whether classList exists, and if so use that
 // rather than the className property that is being used here.
 
 class _CssClassSet implements Set<String> {
 
   final _element;
 
@@ skipping 86 matching lines @@
   /**
    * Helper method used to modify the set of css classes on this element.
    *
    *   f - callback with:
    *      s - a Set of all the css class name currently on this element.
    *
    *   After f returns, the modified set is written to the
    *       className property of this element.
    */
   void _modify( f(Set<String> s)) {
+    assert(!_inMeasurementFrame || !_nodeInDocument(_element));
     Set<String> s = _read();
     f(s);
     _write(s);
   }
 
   /**
    * Read the class names from the HTMLElement class property,
    * and put them into a set (duplicates are discarded).
    */
   Set<String> _read() {
@@ skipping 12 matching lines @@
    * Read the class names as a space-separated string. This is meant to be
    * overridden by subclasses.
    */
   String _className() => _element.className;
 
   /**
    * Join all the elements of a set into one string and write
    * back to the element.
    */
   void _write(Set s) {
+    assert(!_inMeasurementFrame || !_nodeInDocument(_element));
     _element.className = _formatSet(s);
   }
 
   String _formatSet(Set<String> s) {
     // TODO(mattsh) should be able to pass Set to String.joins http:/b/5398605
     List list = new List.from(s);
     return Strings.join(list, ' ');
   }
 
 }