Restructure outline shaker tests.

pkg/front_end/testcases/shaker/transitive_mixins_lib.dart

Index: pkg/front_end/testcases/shaker/transitive_mixins_lib.dart
diff --git a/pkg/front_end/testcases/shaker/implicit_constructors.dart b/pkg/front_end/testcases/shaker/transitive_mixins_lib.dart
similarity index 73%
copy from pkg/front_end/testcases/shaker/implicit_constructors.dart
copy to pkg/front_end/testcases/shaker/transitive_mixins_lib.dart
index 1c3bd887eb8b4dcee9d1a53b938247b0a3dcc6ab..d4c0191357dbdfc8ba8d5c1313de76c9c6e0a3e5 100644
--- a/pkg/front_end/testcases/shaker/implicit_constructors.dart
+++ b/pkg/front_end/testcases/shaker/transitive_mixins_lib.dart
@@ -2,8 +2,10 @@
 // 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 {}
+
+class C extends Object with A implements B {}
+
+class D {}