Import rasta tests.

pkg/fasta/test/rasta/README.md

Index: pkg/fasta/test/rasta/README.md
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+<!--
+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 file.
+-->
+# Rasta regression tests
+
+The tests in this directory were copied from [rasta](https://github.com/dart-lang/rasta/tree/dace1ed5964953ce53065fd41eebfa8920419ff0/test/kernel/regression), at commit [dace1ed](https://github.com/dart-lang/rasta/commit/dace1ed5964953ce53065fd41eebfa8920419ff0).
+
+The rasta expectation files `*.dart.txt` have been copied three times to `* .outline.expect`, `*.direct.expect`, and `*.dart.dartk.expect`.