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

pkg/template_binding/lib/src/template_iterator.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.
 
 part of template_binding;
 
 // This code is a port of what was formerly known as Model-Driven-Views, now
 // located at:
 //     https://github.com/polymer/TemplateBinding
 //     https://github.com/polymer/NodeBind
@@ skipping 412 matching lines @@
 
         var model = _iteratedValue[addIndex];
         var fragment = null;
         var instance = instanceCache.remove(model);
         List instanceBindings;
         List instanceNodes = null;
         if (instance != null && instance.nodes.isNotEmpty) {
           instanceBindings = instance.instanceBindings;
           instanceNodes = instance.nodes;
         } else {
-          instanceBindings = [];
-          if (_instanceModelFn != null) {
-            model = _instanceModelFn(model);
-          }
-          if (model != null) {
-            fragment = _templateExt.createInstance(model, delegate,
-                instanceBindings);
+          try {
+            instanceBindings = [];
+            if (_instanceModelFn != null) {
+              model = _instanceModelFn(model);
+            }
+            if (model != null) {
+              fragment = _templateExt.createInstance(model, delegate,
+                  instanceBindings);
+            }
+          } catch (e, s) {
+            // Dart note: we propagate errors asynchronously here to avoid
+            // disrupting the rendering flow. This is different than in the JS
+            // implementation but it should probably be fixed there too. Dart
+            // hits this case more because non-existing properties in
+            // [PropertyPath] are treated as errors, while JS treats them as
+            // null/undefined.
+            // TODO(sigmund): this should be a synchronous throw when this is
+            // called from createInstance, but that requires enough refactoring
+            // that it should be done upstream first. See dartbug.com/17789.
+            new Completer().completeError(e, s);
           }
         }
 
         _insertInstanceAt(addIndex, fragment, instanceNodes, instanceBindings);
       }
     }
 
     for (var instance in instanceCache.values) {
       _closeInstanceBindings(instance.instanceBindings);
     }
@@ skipping 70 matching lines @@
     _closed = true;
   }
 }
 
 // Dart note: the JavaScript version just puts an expando on the array.
 class _BoundNodes {
   final List<Node> nodes;
   final List<Bindable> instanceBindings;
   _BoundNodes(this.nodes, this.instanceBindings);
 }