Add null literals to polymer expressions

pkg/polymer_expressions/lib/eval.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.eval;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 
@@ skipping 451 matching lines @@
 
   BinaryObserver(BinaryOperator expr, this.left, this.right)
       : super(expr);
 
   String get operator => _expr.operator;
 
   _updateSelf(Scope scope) {
     var f = _BINARY_OPERATORS[operator];
     if (operator == '&&' || operator == '||') {
       _value = f(_toBool(left._value), _toBool(right._value));
+    } else if (operator == '==' || operator == '!=') {
+      _value = f(left._value, right._value);
+    } else if (left._value == null || right._value == null) {
+      _value = null;
     } else {
-      _value = (left._value == null || right._value == null)
-          ? null : f(left._value, right._value);
+      _value = f(left._value, right._value);
     }
   }
 
   accept(Visitor v) => v.visitBinaryOperator(this);
 
 }
 
 class InvokeObserver extends ExpressionObserver<Invoke> implements Invoke {
   final ExpressionObserver receiver;
   List<ExpressionObserver> arguments;
@@ skipping 125 matching lines @@
   Method(this.mirror, this.symbol);
 
   dynamic call(List args) => mirror.invoke(symbol, args, null).reflectee;
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }