Parse parenthesized expressions.

dart/frog/leg/ssa/builder.dart

Index: dart/frog/leg/ssa/builder.dart
diff --git a/dart/frog/leg/ssa/builder.dart b/dart/frog/leg/ssa/builder.dart
index 9c9a4753d39f7dee3514932dd1bd67229035b049..49443ed1d36c56db55c3783b94b19a038a9116f4 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -512,6 +512,10 @@ class SsaBuilder implements Visitor {
     }
   }
 
+  void visitParenthesizedExpression(ParenthesizedExpression node) {
+    visit(node.expression);
+  }
+
   visitOperator(Operator node) {
     // Operators are intercepted in their surrounding Send nodes.
     unreachable();