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

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

Committed: https://code.google.com/p/dart/source/detail?r=30121

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

https://codereview.chromium.org/61793008/diff/1/pkg/observe/lib/src/compound_...
File pkg/observe/lib/src/compound_path_observer.dart (right):

https://codereview.chromium.org/61793008/diff/1/pkg/observe/lib/src/compound_...
pkg/observe/lib/src/compound_path_observer.dart:88: if (_observers.isEmpty)
return;
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).

[Siggi Cherem (dart-lang), 2013-11-08 19:02:42]

Committed patchset #1 manually as r30121 (presubmit successful).