Fix in compound path observer: this fixes the error we saw in our manual testing

pkg/observe/lib/src/compound_path_observer.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.
 
 library observe.src.compound_path_observer;
 
 import 'dart:async';
 import 'package:observe/observe.dart';
 
 /**
@@ skipping 67 matching lines @@
   // TODO(rafaelw): Consider having a seperate ChangeSummary for
   // CompoundBindings so to excess dirtyChecks.
   void _scheduleResolve(_) {
     if (_scheduled) return;
     _scheduled = true;
     scheduleMicrotask(_resolve);
   }
 
   void _resolve() {
     _scheduled = false;
+    if (_observers.isEmpty) return;

[Siggi Cherem (dart-lang), 2013/11/08 18:37:00]

turns out that the schedule happens when there were observers, we end up closing
this object afterwards, but this _resolve is still pending.

_computeValue below makes some assumptions about the list, which are broken if
the _observers list changed. 

In the particular case of TodoMVC, '_computeValue' is defined by
_TemplateIterator, and it assumes it has the entries for the modelValue and the
ifValue (see template_iterator.dart#283).

     var newValue = _observers.map((o) => o.value).toList();
     if (_computeValue != null) newValue = _computeValue(newValue);
     _value = notifyPropertyChange(#value, _value, newValue);
   }
 
   /**
    * Closes the observer.
    *
    * This happens automatically if the [value] property is no longer observed,
    * but this can also be called explicitly.
    */
   void close() {
     if (_observers.isEmpty) return;
 
     if (_started) {
       for (StreamSubscription sub in _subs) {
         sub.cancel();
       }
     }
     _observers.clear();
     _subs.clear();
     _value = null;
   }
 
   observed() => start();
   unobserved() => close();
 }