Fix small bug in RecursiveVisitor

pkg/polymer_expressions/test/visitor_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 visitor_test;
+
+import 'package:polymer_expressions/parser.dart';
+import 'package:polymer_expressions/visitor.dart';
+import 'package:unittest/unittest.dart';
+
+main() {
+
+  group('visitor', () {
+
+    // regression test
+    test('should not infinitely recurse on parenthesized expressions', () {
+      var visitor = new TestVisitor();
+      var expr = new Parser('(1)').parse();
+      visitor.visit(expr);
+    });
+
+  });
+}
+
+class TestVisitor extends RecursiveVisitor {
+  visitExpression(e) => null;
+}