Restructure outline shaker tests.

pkg/front_end/testcases/shaker/transitive_class_constructor_lib.dart

 // Copyright (c) 2017, the Dart project authors. Please see the AUTHORS file
 // 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 A1 {}
 
-/// Tree-shaker preserves APIs used anywhere in the library, there is no special
-/// root, and `main` is no special.
-class X {
-  static foo() => toplevel();
+class A2 {}
+
+class A3 {}
+
+class A4 {}
+
+class A5 {}
+
+class A6 {}
+
+class B {
+  A1 publicField;
+  A2 _privateField;
+
+  B();
+  B.publicConstructor(A3 a, [A4 b]);
+  B._privateConstructor(A5 a);
 }