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

tests/compiler/dart2js/analyze_dart_test.dart

Index: tests/compiler/dart2js/analyze_dart_test.dart
diff --git a/tests/compiler/dart2js/analyze_strong_test.dart b/tests/compiler/dart2js/analyze_dart_test.dart
similarity index 90%
rename from tests/compiler/dart2js/analyze_strong_test.dart
rename to tests/compiler/dart2js/analyze_dart_test.dart
index 5b5fbd14566e9eb2c98a7b1cbfb10961c06214f4..6d3f559d3542e5a0f991f34f74cb0970887b6362 100644
--- a/tests/compiler/dart2js/analyze_strong_test.dart
+++ b/tests/compiler/dart2js/analyze_dart_test.dart
@@ -8,7 +8,7 @@ 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"
   ]);