Add null literals to polymer expressions

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 29 matching lines @@
     test('should return a literal string', () {
       expectEval('"hello"', "hello");
       expectEval("'hello'", "hello");
     });
 
     test('should return a literal boolean', () {
       expectEval('true', true);
       expectEval('false', false);
     });
 
+    test('should return a literal null', () {
+      expectEval('null', null);
+    });
+
     test('should return a literal map', () {
       expectEval('{"a": 1}', equals(new Map.from({'a': 1})));
       expectEval('{"a": 1}', containsPair('a', 1));
     });
 
     test('should call methods on a literal map', () {
       expectEval('{"a": 1}.length', 1);
     });
 
     test('should evaluate unary operators', () {
       expectEval('+a', 2, null, {'a': 2});
       expectEval('-a', -2, null, {'a': 2});
       expectEval('!a', false, null, {'a': true});
     });
 
     test('should evaluate binary operators', () {
       expectEval('1 + 2', 3);
       expectEval('2 - 1', 1);
       expectEval('4 / 2', 2);
       expectEval('2 * 3', 6);
 
       expectEval('1 == 1', true);
       expectEval('1 == 2', false);
+      expectEval('1 == null', false);
       expectEval('1 != 1', false);
       expectEval('1 != 2', true);
+      expectEval('1 != null', true);
 
       expectEval('1 > 1', false);
       expectEval('1 > 2', false);
       expectEval('2 > 1', true);
       expectEval('1 >= 1', true);
       expectEval('1 >= 2', false);
       expectEval('2 >= 1', true);
       expectEval('1 < 1', false);
       expectEval('1 < 2', true);
       expectEval('2 < 1', false);
@@ skipping 70 matching lines @@
     test('should return null if null is invoked', () {
       expectEval('a()', null, null, {'a': null});
     });
 
     test('should return null if an operand is null', () {
       expectEval('a + b', null, null, {'a': null, 'b': null});
       expectEval('+a', null, null, {'a': null});
     });
 
     test('should treat null as false', () {
+      expectEval('!null', true);
+      expectEval('true && null', false);
+      expectEval('null || false', false);
+
       expectEval('!a', true, null, {'a': null});
 
       expectEval('a && b', false, null, {'a': null, 'b': true});
       expectEval('a && b', false, null, {'a': true, 'b': null});
       expectEval('a && b', false, null, {'a': null, 'b': false});
       expectEval('a && b', false, null, {'a': false, 'b': null});
       expectEval('a && b', false, null, {'a': null, 'b': null});
 
       expectEval('a || b', true, null, {'a': null, 'b': true});
       expectEval('a || b', true, null, {'a': true, 'b': null});
@@ skipping 194 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");
   })]);
 }