Don’t throw on assignments to non-assignable expressions. Allow non-literal indices in assignable e…

pkg/polymer_expressions/lib/polymer_expressions.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.
 
 /**
  * A binding delegate used with Polymer elements that
  * allows for complex binding expressions, including
  * property access, function invocation,
  * list/map indexing, and two-way filtering.
  *
@@ skipping 281 matching lines @@
 
   get value {
     // if there's a callback, then _value has been set, if not we need to
     // force an evaluation
     if (_callback != null) return _value;
     return _oneTime(_expr, _scope, _converter);
   }
 
   set value(v) {
     try {
-      var newValue = assign(_expr, v, _scope);
+      var newValue = assign(_expr, v, _scope, checkAssignability: false);
       _check(newValue, skipChanges: true);
     } catch (e, s) {
       new Completer().completeError(
           "Error evaluating expression '$_expr': $e", s);
     }
   }
 
   Object open(callback(value)) {
     if (_callback != null) throw new StateError('already open');
 
@@ skipping 30 matching lines @@
  * Factory function used for testing.
  */
 class ScopeFactory {
   const ScopeFactory();
   modelScope({Object model, Map<String, Object> variables}) =>
       new Scope(model: model, variables: variables);
 
   childScope(Scope parent, String name, Object value) =>
       parent.childScope(name, value);
 }