Remove AsyncError with Expando.

tools/dom/src/ModelTreeObserver.dart

 // Copyright (c) 2013, 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.
 
 part of html;
 
 class _ModelTreeObserver {
   static bool _initialized = false;
 
   /**
@@ skipping 65 matching lines @@
    */
   static void propagateModel(Node node, model, bool fullTree) {
     // Calling into user code with the != call could generate exceptions.
     // Catch and report them a global exceptions.
     try {
       if (node._hasLocalModel != true && node._model != model &&
           node._modelChangedStream != null) {
         node._model = model;
         node._modelChangedStream.add(node);
       }
-    } on AsyncError catch (e) {
-      e.throwDelayed();
     } catch (e, s) {
-      new AsyncError(e, s).throwDelayed();
+      new Future.immediateError(e, s);
     }
     for (var child = node.$dom_firstChild; child != null;
         child = child.nextNode) {
       if (child._hasLocalModel != true) {
         propagateModel(child, model, fullTree);
       } else if (fullTree) {
         propagateModel(child, child._model, true);
       }
     }
   }
 }