Add null literals to polymer expressions

pkg/polymer_expressions/lib/parser.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.parser;
 
 import 'tokenizer.dart';
 import 'expression.dart';
 
 const _UNARY_OPERATORS = const ['+', '-', '!'];
@@ skipping 178 matching lines @@
 
   Expression _parseInvokeOrIdentifier() {
     if (_token.value == 'true') {
       _advance();
       return _astFactory.literal(true);
     }
     if (_token.value == 'false') {
       _advance();
       return _astFactory.literal(false);
     }
+    if (_token.value == 'null') {
+      _advance();
+      return _astFactory.literal(null);
+    }
     var identifier = _parseIdentifier();
     var args = _parseArguments();
     if (args == null) {
       return identifier;
     } else {
       return _astFactory.invoke(identifier, null, args);
     }
   }
 
   Invoke _parseInvoke() {
@@ skipping 57 matching lines @@
     return value;
   }
 
   Literal<double> _parseDecimal([String prefix = '']) {
     var value = _astFactory.literal(double.parse('$prefix${_token.value}'));
     _advance();
     return value;
   }
 
 }