PolymerExpressions: fix for null iterators in comprehensions

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 'package:polymer_expressions/eval.dart';
 import 'package:polymer_expressions/filter.dart';
@@ skipping 179 matching lines @@
     test('should evaluate an "in" expression', () {
       var scope = new Scope(variables: {'items': [1, 2, 3]});
       var comprehension = eval(parse('item in items'), scope);
       expect(comprehension.iterable, orderedEquals([1, 2, 3]));
     });
 
     test('should handle null iterators in "in" expressions', () {
       var scope = new Scope(variables: {'items': null});
       var comprehension = eval(parse('item in items'), scope);
       expect(comprehension, isNotNull);
-      expect(comprehension.iterable, null);
+      expect(comprehension.iterable, []]);
     });
 
   });
 
   group('assign', () {
 
     test('should assign a single identifier', () {
       var foo = new Foo(name: 'a');
       assign(parse('name'), 'b', new Scope(model: foo));
       expect(foo.name, 'b');
@@ skipping 168 matching lines @@
     expect(value, afterMatcher);
     expect(observer.currentValue, afterMatcher);
     passed = true;
   });
   mutate();
   // fail if we don't receive an update by the next event loop
   return Future.wait([future, new Future(() {
     expect(passed, true, reason: "Didn't receive a change notification on $s");
   })]);
 }