Add null literals to polymer expressions

pkg/polymer_expressions/lib/eval.dart

Index: pkg/polymer_expressions/lib/eval.dart
diff --git a/pkg/polymer_expressions/lib/eval.dart b/pkg/polymer_expressions/lib/eval.dart
index 7ed7b5a5afb9ec4364b662554ec9695cb4cf9444..9d9f8ef8f5d5ccb30d883cdea4c0a08fe4ce7c07 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -469,9 +469,12 @@ class BinaryObserver extends ExpressionObserver<BinaryOperator>
     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);
     }
   }