Fix model-tree-observer in html.

tools/dom/templates/html/impl/impl_Node.darttemplate

 // 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.
 
 part of $LIBRARYNAME;
 
 /**
  * Lazy implementation of the child nodes of an element that does not request
  * the actual child nodes of an element until strictly necessary greatly
  * improving performance for the typical cases where it is not required.
@@ skipping 281 matching lines @@
    * nodes.
    *
    * Currently this does not support propagation through Shadow DOMs.
    *
    * [clearModel] must be used to remove the model property from this node
    * and have the model inherit from ancestor nodes.
    */
   @Experimental
   get model {
     // If we have a change handler then we've cached the model locally.
-    if (!_modelChangedStreams.isEmpty) {
+    if (_modelChangedStreams != null && !_modelChangedStreams.isEmpty) {
       return _model;
     }
     // Otherwise start looking up the tree.
     for (var node = this; node != null; node = node.parentNode) {
       if (node._hasLocalModel == true) {
         return node._model;
       }
     }
     return null;
   }
 
   @Experimental
   void set model(value) {
     var changed = model != value;
     _model = value;
     _hasLocalModel = true;
     _ModelTreeObserver.initialize();
 
     if (changed) {
-      if (!_modelChangedStreams.isEmpty) {
+      if (_modelChangedStreams != null && !_modelChangedStreams.isEmpty) {
         _modelChangedStreams.toList().forEach((stream) => stream.add(this));
       }
       // Propagate new model to all descendants.
       _ModelTreeObserver.propagateModel(this, value, false);
     }
   }
 
   /**
    * Clears the locally set model and makes this model be inherited from parent
    * nodes.
    */
   @Experimental
   void clearModel() {
     if (_hasLocalModel == true) {
       _hasLocalModel = false;
 
       // Propagate new model to all descendants.
       if (parentNode != null) {
         _ModelTreeObserver.propagateModel(this, parentNode.model, false);
       } else {
         _ModelTreeObserver.propagateModel(this, null, false);
       }
     }
   }
 
   /**
    * Get a stream of models, whenever the model changes.
    */
   Stream<Node> get onModelChanged {
-    var result;
-    result = new StreamController(
-        onListen: () { _modelChangedStreams.add(result); },
-        onCancel: () { _modelChangedStreams.remove(result); });
-    return result;
+    if (_modelChangedStreams == null) {
+      _modelChangedStreams = new Set<StreamController<Node>>();
+    }
+    var controller;
+    controller = new StreamController(
+        onListen: () { _modelChangedStreams.add(controller); },
+        onCancel: () { _modelChangedStreams.remove(controller); });
+    return controller.stream;
   }
 
   /**
    * Print out a String representation of this Node.
    */
   String toString() => localName == null ?
       (nodeValue == null ? super.toString() : nodeValue) : localName;
 
 $!MEMBERS
 }