Reapply CL: first step for modular support in fasta

pkg/front_end/testcases/shaker/roots6.dart

Index: pkg/front_end/testcases/shaker/roots6.dart
diff --git a/pkg/front_end/testcases/shaker/roots6.dart b/pkg/front_end/testcases/shaker/roots6.dart
new file mode 100644
index 0000000000000000000000000000000000000000..175535c408de952926dc48994793def5d38cb3c0
--- /dev/null
+++ b/pkg/front_end/testcases/shaker/roots6.dart
@@ -0,0 +1,11 @@
+// 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';
+
+/// Tree-shaker preserves APIs used anywhere in the library, there is no special
+/// root, and `main` is no special.
+class X {
+  static var s = toplevel();
+}