Add null literals to polymer expressions

pkg/polymer_expressions/test/parser_test.dart

Index: pkg/polymer_expressions/test/parser_test.dart
diff --git a/pkg/polymer_expressions/test/parser_test.dart b/pkg/polymer_expressions/test/parser_test.dart
index 21b11fb786147f36248ad53fbfb4218e8adbf47d..66c1d046b58c699acd69bc7f06f5d055e4c19a5c 100644
--- a/pkg/polymer_expressions/test/parser_test.dart
+++ b/pkg/polymer_expressions/test/parser_test.dart
@@ -27,6 +27,15 @@ main() {
       expectParse('"abc"', literal('abc'));
     });
 
+    test('should parse a bool literal', () {
+      expectParse('true', literal(true));
+      expectParse('false', literal(false));
+    });
+
+    test('should parse a null literal', () {
+      expectParse('null', literal(null));
+    });
+
     test('should parse an integer literal', () {
       expectParse('123', literal(123));
     });