Make invalid to override method names for local nested functions

tests/compiler/dart2js_extra/immutable_nested_closure_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-@notExisting
-var x;
-
+// Regression test for dartbug.com/19129
 main() {
-  print(x);
+  m() {
+    m = () => 0; /// 01: runtime error
+  }
+  m();
 }