Create language test that exercises bug #29709.

tests/language/part_refers_to_core_library_test.dart

Index: tests/language/part_refers_to_core_library_test.dart
diff --git a/tests/language/part_refers_to_core_library_test.dart b/tests/language/part_refers_to_core_library_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..0d961c090fc14376f1b63a651f182114477bd4fb
--- /dev/null
+++ b/tests/language/part_refers_to_core_library_test.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 file.
+
+// This test reproduces https://github.com/dart-lang/sdk/issues/29709.
+
+library dart.async;
+
+part 'dart:async/future.dart'; /// 01: compile-time error
+
+main() {}