Restructure outline shaker tests.

pkg/front_end/testcases/shaker/target_top_function.dart

Index: pkg/front_end/testcases/shaker/target_top_function.dart
diff --git a/pkg/front_end/testcases/shaker/classes_in_signatures.dart b/pkg/front_end/testcases/shaker/target_top_function.dart
similarity index 70%
rename from pkg/front_end/testcases/shaker/classes_in_signatures.dart
rename to pkg/front_end/testcases/shaker/target_top_function.dart
index ded6f0a32b30b375fd94e88169640054399f641f..8c366c0c105da33adafe20e56dfc33ede201f64d 100644
--- a/pkg/front_end/testcases/shaker/classes_in_signatures.dart
+++ b/pkg/front_end/testcases/shaker/target_top_function.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 'lib/targets_top.dart';
 
-/// We preserve K because it is mentioned in a type.
-K foo() => null;
+var v = myTopLevelFunction();