Restructure outline shaker tests.

pkg/front_end/testcases/shaker/transitive_superclass_lib.dart

Index: pkg/front_end/testcases/shaker/transitive_superclass_lib.dart
diff --git a/pkg/front_end/testcases/shaker/implicit_constructors.dart b/pkg/front_end/testcases/shaker/transitive_superclass_lib.dart
similarity index 79%
rename from pkg/front_end/testcases/shaker/implicit_constructors.dart
rename to pkg/front_end/testcases/shaker/transitive_superclass_lib.dart
index 1c3bd887eb8b4dcee9d1a53b938247b0a3dcc6ab..c89598bc1729a6473c7087db2666b49faeec432e 100644
--- a/pkg/front_end/testcases/shaker/implicit_constructors.dart
+++ b/pkg/front_end/testcases/shaker/transitive_superclass_lib.dart
@@ -2,8 +2,8 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
-import 'lib/lib.dart';
+class _A {}
 
-class D extends F {
-  D.x();
-}
+class B extends _A {}
+
+class C extends B {}