Remove corelib/corelib_strong and migrate last two remaining tests.

tools/gardening/test/parse_build_results_test.dart

 // Copyright (c) 2017, 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 'package:args/args.dart';
 import 'package:expect/expect.dart';
 import 'package:gardening/src/buildbot_structures.dart';
 import 'package:gardening/src/util.dart';
 
 import 'test_client.dart';
@@ skipping 17 matching lines @@
       if (s.config.testName == testName) {
         status = s;
         break;
       }
     }
     Expect.isNotNull(status, "TestStatus for '$testName' not found.");
     Expect.equals(
         expectedStatus, status.status, "Unexpected status for '$testName'.");
   }
 
-  checkTest('corelib/list_growable_test', 'pass');
   checkTest('corelib_2/map_keys2_test', 'fail');
   client.close();
 }