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

pkg/observe/lib/src/compound_path_observer.dart

Index: pkg/observe/lib/src/compound_path_observer.dart
diff --git a/pkg/observe/lib/src/compound_path_observer.dart b/pkg/observe/lib/src/compound_path_observer.dart
index 6fee6897cd24b45b5bbec4231e48cba4172563b0..1594e082c5b1bbddc5efe12f488454675eb06600 100644
--- a/pkg/observe/lib/src/compound_path_observer.dart
+++ b/pkg/observe/lib/src/compound_path_observer.dart
@@ -85,6 +85,7 @@ class CompoundPathObserver extends ChangeNotifier {
 
   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);