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

pkg/template_binding/test/node_bind_test.dart

Index: pkg/template_binding/test/node_bind_test.dart
diff --git a/pkg/template_binding/test/node_bind_test.dart b/pkg/template_binding/test/node_bind_test.dart
index 93bcd8fe2ff1fea8cdd5148e366ab5516552a853..cfe8d7cc6abeb4f2ec214c1cade6152b4eb2c657 100644
--- a/pkg/template_binding/test/node_bind_test.dart
+++ b/pkg/template_binding/test/node_bind_test.dart
@@ -70,8 +70,9 @@ testBindings() {
   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', () {