Restructure outline shaker tests.

pkg/front_end/testcases/shaker/source_class.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';
+import 'lib/sources.dart';
 
-/// Tree-shaker preserves APIs used anywhere in the library, there is no special
-/// root, and `main` is no special.
-class X {
-  var s = toplevel();
+class C {
+  A1 f1;
+  var f2 = A2;
+
+  A3 m1(A4 a, [A5 b]) => null;
+  A3 m2(A4 a, {A6 b}) => null;
+
+  A7 get getter => null;
+  void set setter(A8 v) {}
 }