Switch the defaults: strong mode is now default, but we stil check tests in Dart mode.

tests/compiler/dart2js/analyze_dart_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:testing/src/run_tests.dart" as testing show main;
 
 main() {
   // This method is async, but keeps a port open to prevent the VM from exiting
   // prematurely.
   return testing.main(<String>[
-    "--config=pkg/compiler/testing_strong.json",
+    "--config=pkg/compiler/testing_dart.json",
     "--verbose",
     "analyze"
   ]);
 }