Resolve initializers in their proper scope.

dart/frog/leg/tree/nodes.dart

Index: dart/frog/leg/tree/nodes.dart
diff --git a/dart/frog/leg/tree/nodes.dart b/dart/frog/leg/tree/nodes.dart
index 4b4b74cbcdac1ca0dafc32525eb80ddc8a445254..0fb6093f8fea47146e5d7cbb5ea4ea3b63a573d6 100644
--- a/dart/frog/leg/tree/nodes.dart
+++ b/dart/frog/leg/tree/nodes.dart
@@ -1628,3 +1628,12 @@ class IsInterpolationVisitor extends AbstractVisitor<bool> {
   bool visitStringJuxtaposition(StringJuxtaposition node)
       => node.isInterpolation;
 }
+
+/**
+ * If the given node is a send set, it visits its initializer (first
+ * argument).
+ */
+initializerDo(Node node, f(Node node)) {
+  SendSet send = node.asSendSet();
+  if (send !== null) return f(send.arguments.head);
+}