Fix bug with re-scoping arrow function parameter initializers

src/ast/scopes.h

Index: src/ast/scopes.h
diff --git a/src/ast/scopes.h b/src/ast/scopes.h
index cb1fb5bdac5659f2aa71ffd4bb074e227e3546fa..e420cfe053ab4eb616020747c54d23f592715424 100644
--- a/src/ast/scopes.h
+++ b/src/ast/scopes.h
@@ -216,7 +216,11 @@ class Scope: public ZoneObject {
   // Adds a temporary variable in this scope's TemporaryScope. This is for
   // adjusting the scope of temporaries used when desugaring parameter
   // initializers.
-  void AddTemporary(Variable* var) { temps_.Add(var, zone()); }
+  void AddTemporary(Variable* var) {
+    // Temporaries are only placed in ClosureScopes.
+    DCHECK_EQ(ClosureScope(), this);
+    temps_.Add(var, zone());
+  }
 
   // Adds the specific declaration node to the list of declarations in
   // this scope. The declarations are processed as part of entering