Restructure outline shaker tests.

pkg/front_end/testcases/shaker/transitive_class_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.
+
+class A1 {}
+
+class A2 {}
+
+class A3 {}
+
+class A4 {}
+
+class A5 {}
+
+class A6 {}
+
+class A7 {}
+
+class A8 {}
+
+class A9 {}
+
+class B {
+  A1 publicField;
+  A2 _privateField;
+}
+
+class C {
+  A3 publicField;
+  A4 _privateField;
+  B b;
+
+  C();
+  C.named();
+
+  void publicMethod(A5 a) {}
+  void _privateMethod(A6 a) {}
+}