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

pkg/polymer_expressions/lib/eval.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 polymer_expressions.eval;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:observe/observe.dart';
@@ skipping 132 matching lines @@
 }
 
 
 /**
  * A scope in polymer expressions that can map names to objects. Scopes contain
  * a set of named variables and a unique model object. The scope structure
  * is then used to lookup names using the `[]` operator. The lookup first
  * searches for the name in local variables, then in global variables,
  * and then finally looks up the name as a property in the model.
  */
-abstract class Scope {
+abstract class Scope implements Indexable<String, Object> {
   Scope._();
 
   /** Create a scope containing a [model] and all of [variables]. */
   factory Scope({Object model, Map<String, Object> variables}) {
     var scope = new _ModelScope(model);
     return variables == null ? scope
         : new _GlobalsScope(new Map<String, Object>.from(variables), scope);
   }
 
   /** Return the unique model in this scope. */
   Object get model;
 
   /**
    * Lookup the value of [name] in the current scope. If [name] is 'this', then
    * we return the [model]. For any other name, this finds the first variable
    * matching [name] or, if none exists, the property [name] in the [model].
    */
-  Object operator[](String name);
+  Object operator [](String name);
+
+  operator []=(String name, Object value) {
+    throw new UnsupportedError('[]= is not supported in Scope.');
+  }
 
   /**
    * Returns whether [name] is defined in [model], that is, a lookup
    * would not find a variable with that name, but there is a non-null model
    * where we can look it up as a property.
    */
   bool _isModelProperty(String name);
 
   /** Create a new scope extending this scope with an additional variable. */
   Scope childScope(String name, Object value) =>
@@ skipping 558 matching lines @@
 
   Comprehension(this.identifier, Iterable iterable)
       : iterable = (iterable != null) ? iterable : const [];
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }