Parse parenthesized expressions.

dart/frog/leg/scanner/listener.dart

Index: dart/frog/leg/scanner/listener.dart
diff --git a/dart/frog/leg/scanner/listener.dart b/dart/frog/leg/scanner/listener.dart
index 683cb6fbbc13bc0ba81e8877da31a5775fc17c9c..2abe873a8586785f3866b8095d984aa72de4f493 100644
--- a/dart/frog/leg/scanner/listener.dart
+++ b/dart/frog/leg/scanner/listener.dart
@@ -209,6 +209,9 @@ class Listener {
   void handleNoTypeVariables(Token token) {
   }
 
+  void handleParenthesizedExpression(BeginGroupToken token) {
+  }
+
   void handleUnaryPostfixExpression(Token token) {
   }
 
@@ -322,6 +325,11 @@ class ElementListener extends Listener {
     }
   }
 
+  void handleParenthesizedExpression(BeginGroupToken token) {
+    Expression expression = popNode();
+    pushNode(new ParenthesizedExpression(expression, token));
+  }
+
   Token expected(String string, Token token) {
     canceler.cancel("Expected '$string', but got '$token'", token: token);
   }
@@ -518,7 +526,7 @@ class NodeListener extends ElementListener {
   void endIfStatement(Token ifToken, Token elseToken) {
     Statement elsePart = (elseToken === null) ? null : popNode();
     Statement thenPart = popNode();
-    NodeList condition = popNode();
+    ParenthesizedExpression condition = popNode();
     pushNode(new If(condition, thenPart, elsePart, ifToken, elseToken));
   }