Remove corelib/corelib_strong and migrate last two remaining tests.

tools/testing/dart/options.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:io';
 
 import 'configuration.dart';
 import 'path.dart';
 import 'runtime_updater.dart';
 import 'utils.dart';
 
 const _defaultTestSelectors = const [
   'samples',
   'standalone',
-  'corelib',
   'corelib_2',
   'co19',
   'language',
   'language_2',
   'isolate',
   'vm',
   'html',
   'benchmark_smoke',
   'utils',
   'lib',
@@ skipping 730 matching lines @@
       if (name == option.name) return option;
 
       // Allow hyphens instead of underscores as the separator since they are
       // more common for command line flags.
       if (name == option.name.replaceAll("_", "-")) return option;
     }
 
     return null;
   }
 }