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

tests/compiler/dart2js/analyze_strong_test.dart

Index: tests/compiler/dart2js/analyze_strong_test.dart
diff --git a/tests/compiler/dart2js/analyze_strong_test.dart b/tests/compiler/dart2js/analyze_strong_test.dart
deleted file mode 100644
index 5b5fbd14566e9eb2c98a7b1cbfb10961c06214f4..0000000000000000000000000000000000000000
--- a/tests/compiler/dart2js/analyze_strong_test.dart
+++ /dev/null
@@ -1,15 +0,0 @@
-// 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"
-  ]);
-}