Migrate block 39 language/async_backwards ... /async_star_take.

tests/language_strong/async_rethrow_test.dart

-// Copyright (c) 2015, 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.
-
-import "dart:async";
-import "package:expect/expect.dart";
-
-const String exceptionString = "exceptionalString";
-
-throwString() async {
-  try {
-    throw exceptionString;
-  } catch (e) {
-    await 1;
-    throw e;
-  }
-}
-
-rethrowString() async {
-  try {
-    throw exceptionString;
-  } catch (e) {
-    await 1;
-    rethrow;
-  }
-}
-
-testThrow() {
-  Future f = throwString();
-  f.then((v) {
-    Expect.fail("Exception not thrown");
-  }, onError: (e) {
-    Expect.equals(exceptionString, e);
-  });
-}
-
-testRethrow() {
-  Future f = rethrowString();
-  f.then((v) {
-    Expect.fail("Exception not thrown");
-  }, onError: (e) {
-    Expect.equals(exceptionString, e);
-  });
-}
-
-main() {
-  testThrow();
-  testRethrow();
-}