Remove corelib/corelib_strong and migrate last two remaining tests.

tests/corelib_strong/symbol_map_helper.dart

-// Copyright (c) 2012, 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.
-
-library dart.test.symbol_map_helper;
-
-// TODO(ahe): Update map literals to avoid this method.
-Map<Symbol, dynamic> symbolMapToStringMap(Map<String, dynamic> map) {
-  if (map == null) return null;
-  Map<Symbol, dynamic> result = new Map<Symbol, dynamic>();
-  map.forEach((name, value) {
-    result[new Symbol(name)] = value;
-  });
-  return result;
-}