Reapply CL: first step for modular support in fasta

pkg/front_end/testcases/shaker/roots0.dart

Index: pkg/front_end/testcases/shaker/roots0.dart
diff --git a/pkg/front_end/testcases/shaker/roots0.dart b/pkg/front_end/testcases/shaker/roots0.dart
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index 0000000000000000000000000000000000000000..3a3c44666d77a3967d55f0978f07a3393ee0de22
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+++ b/pkg/front_end/testcases/shaker/roots0.dart
@@ -0,0 +1,9 @@
+// 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.
+main() => toplevel();