Restructure outline shaker tests.

pkg/front_end/testcases/shaker/source_top.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/sources.dart';
+
+class C1 extends A1 {}
+
+class C2 implements A2 {}
+
+class C3 extends Object with A3 {}
+
+typedef A4 F1(A5 a, [A6 b]);
+typedef A4 F2(A5 a, {A7 b});
+
+A8 topLevelVariable1;
+var topLevelVariable2 = A9;
+
+A10 topLevelFunction1(A11 a, [A12 b]) => null;
+A10 topLevelFunction2(A11 a, {A13 b}) => null;
+
+@Meta(A14)
+class X {}
+
+class Meta {
+  final f;
+  const Meta(this.f);
+}