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

tests/compiler/dart2js/analyze_strong_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",
-    "--verbose",
-    "analyze"
-  ]);
-}