Restructure outline shaker tests.

pkg/front_end/testcases/shaker/transitive_superclass.dart

Index: pkg/front_end/testcases/shaker/transitive_superclass.dart
diff --git a/pkg/front_end/testcases/shaker/transitive_superclass.dart b/pkg/front_end/testcases/shaker/transitive_superclass.dart
index 7266147217a5eecd96c2b970a31962cd327021d6..c2e5d5df141856011a09295a4093efabee19b40d 100644
--- a/pkg/front_end/testcases/shaker/transitive_superclass.dart
+++ b/pkg/front_end/testcases/shaker/transitive_superclass.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_superclass_lib.dart';
 
-/// B and it's superclass _A should be preserved.
-main() => new B();
+C c;