Implement proper scoping for "this" in arrow functions

test/mjsunit/debug-scopes.js

Index: test/mjsunit/debug-scopes.js
diff --git a/test/mjsunit/debug-scopes.js b/test/mjsunit/debug-scopes.js
index 7c08120e2adf82e753117726d253263a9d170966..9d1b38ffcd0cf81275cd2a75f055592aaf08073b 100644
--- a/test/mjsunit/debug-scopes.js
+++ b/test/mjsunit/debug-scopes.js
@@ -167,6 +167,10 @@ function CheckScopeContent(content, number, exec_state) {
   if (!scope.scopeObject().property('arguments').isUndefined()) {
     scope_size--;
   }
+  // Ditto for 'this'.
+  if (!scope.scopeObject().property('this').isUndefined()) {
+    scope_size--;
+  }
   // Skip property with empty name.
   if (!scope.scopeObject().property('').isUndefined()) {
     scope_size--;