Add a separate scope for switch

src/scopes.h

Index: src/scopes.h
diff --git a/src/scopes.h b/src/scopes.h
index 2f600b9f0cbc3aa244a94840e6450a96419a549e..c15e201fab7fab839d7bafcc6fbf24e5f9b49aed 100644
--- a/src/scopes.h
+++ b/src/scopes.h
@@ -246,6 +246,10 @@ class Scope: public ZoneObject {
   //     for (let x ...) stmt
   //   start position: start position of '('
   //   end position: end position of last token of 'stmt'
+  // * For the scope of a switch statement
+  //     switch (tag) { cases }
+  //   start position: start position of '{'
+  //   end position: end position of '}'
   int start_position() const { return start_position_; }
   void set_start_position(int statement_pos) {
     start_position_ = statement_pos;