Restructure outline shaker tests.

pkg/front_end/testcases/shaker/transitive_mixins.dart

Index: pkg/front_end/testcases/shaker/transitive_mixins.dart
diff --git a/pkg/front_end/testcases/shaker/transitive_mixins.dart b/pkg/front_end/testcases/shaker/transitive_mixins.dart
index 461293decb0ad66e428b10f9b3fd144eae962d4f..43ee444340ec2f01dee3bc5b7c8c9f27d4e807c8 100644
--- a/pkg/front_end/testcases/shaker/transitive_mixins.dart
+++ b/pkg/front_end/testcases/shaker/transitive_mixins.dart
@@ -2,7 +2,6 @@
 // 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';
+import 'transitive_mixins_lib.dart';
 
-/// We pull in transitive super types from mixins and interfaces.
-foo(M1 x) => null;
+C v = null;