[mdv] Avoid observing placeholder arrays in TemplateIterator

pkg/observe/lib/src/compound_binding.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 observe;
 
 /** The callback used in the [CompoundBinding.combinator] field. */
 typedef Object CompoundBindingCombinator(Map objects);
 
 /**
@@ skipping 39 matching lines @@
 
   set combinator(CompoundBindingCombinator combinator) {
     _combinator = combinator;
     if (combinator != null) _scheduleResolve();
   }
 
   static const _VALUE = const Symbol('value');
 
   get value => _value;
 
+  int get length => _bindings.length;
+
   void set value(newValue) {
     _value = notifyPropertyChange(_VALUE, _value, newValue);
   }
 
   void bind(name, model, String path) {
     unbind(name);
 
      // TODO(jmesserly): should we avoid observing until we are observed,
      // similar to PathObserver? Similar for unobserving?
     _bindings[name] = new PathObserver(model, path).bindSync((value) {
@@ skipping 36 matching lines @@
     for (var binding in _bindings.values) {
       binding.cancel();
     }
     _bindings.clear();
     _values.clear();
 
     _disposed = true;
     value = null;
   }
 }