Reapply change that makes path-observer more agressive with property lookups.

pkg/template_binding/test/node_bind_test.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 template_binding.test.node_bind_test;
 
 import 'dart:async';
 import 'dart:html';
 
 import 'package:observe/observe.dart'
@@ skipping 52 matching lines @@
   test('No Path', () {
     var text = new Text('hi');
     var model = 1;
     nodeBind(text).bind('text', new PathObserver(model));
     expect(text.text, '1');
   });
 
   test('Path unreachable', () {
     var text = testDiv.append(new Text('hi'));
     var model = 1;
-    nodeBind(text).bind('text', new PathObserver(model, 'a'));
-    expect(text.text, '');
+    var pathObserver = new PathObserver(model, 'a');
+    expect(() => nodeBind(text).bind('text', pathObserver), throws);
+    expect(text.text, 'hi');
   });
 
   test('Observer is Model', () {
     var text = new Text('');
     var model = toObservable({'a': {'b': {'c': 1}}});
     var observer = new PathObserver(model, 'a.b.c');
     nodeBind(text).bind('text', observer);
     expect(text.text, '1');
 
     var binding = nodeBind(text).bindings['text'];
@@ skipping 634 matching lines @@
     }).then(endOfMicrotask).then((_) {
       expect(select.value, 'X');
       expect(model['selected'], 'X');
 
       model['selected'] = 'a';
     }).then(endOfMicrotask).then((_) {
       expect(select.value, 'a');
     });
   });
 }