Silently fail when assigning to properties on null

pkg/polymer_expressions/test/eval_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 eval_test;
 
 import 'dart:async';
 
 // Import mirrors to cause all mirrors to be retained by dart2js.
 // The tests reflect on LinkedHashMap.length and String.length.
@@ skipping 239 matching lines @@
         'parseInt': parseInt,
         'add': add,
       };
       var scope = new Scope(model: foo, variables: globals);
       assign(parse('age | add(7)'), 29, scope);
       expect(foo.age, 22);
       assign(parse('name | parseInt() | add(10)'), 29, scope);
       expect(foo.name, '19');
     });
 
+    test('should throw on assignments to properties on null', () {
+      assign(parse('name'), 'b', new Scope(model: null));
+    });
+
   });
 
   group('scope', () {
     test('should return fields on the model', () {
       var foo = new Foo(name: 'a', age: 1);
       var scope = new Scope(model: foo);
       expect(scope['name'], 'a');
       expect(scope['age'], 1);
     });
 
@@ skipping 130 matching lines @@
   return Future.wait([future, new Future(() {
     expect(passed, true, reason: "Didn't receive a change notification on $s");
   })]);
 }
 
 // Regression test from https://code.google.com/p/dart/issues/detail?id=13459
 class WordElement extends Observable {
   @observable List chars1 = 'abcdefg'.split('');
   @reflectable List filteredList(List original) => [original[0], original[1]];
 }