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

tests/language/async_star_pause_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.
-
-library async_star_pause_test;
-
-import "package:unittest/unittest.dart";
-import "dart:async";
-
-main() {
-  test("await for pauses stream during body", () async {
-    // Assumes await-for uses streamIterator.
-    var log = [];
-    var s = () async* {
-      for (int i = 0; i < 3; i++) {
-        log.add("$i-");
-        yield i;
-        // Should pause here until next iteration of await-for loop.
-        log.add("$i+");
-      }
-    }();
-    await for (var i in s) {
-      log.add("$i?");
-      await nextMicrotask();
-      log.add("$i!");
-    }
-    expect(log, [
-      "0-",
-      "0?",
-      "0!",
-      "0+",
-      "1-",
-      "1?",
-      "1!",
-      "1+",
-      "2-",
-      "2?",
-      "2!",
-      "2+"
-    ]);
-  });
-}
-
-Future nextMicrotask() => new Future.microtask(() {});