Revert "Remove package:unittest from some tests"

tests/language/async_await_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_await_test;
 
-import "package:test/test.dart";
+// Use of package:unittest and package:test is deprecated in sdk/tests.
+// Do not add any more uses of this package.
+import "package:unittest/unittest.dart";
 import "dart:async";
 
 main() {
   bool checkedMode = false;
   assert((checkedMode = true));
 
   group("basic", () {
     test("async w/o await", () {
       f() async { return id(42); }
       return expect42(f());
@@ skipping 1981 matching lines @@
       if (onError is BinaryFunction) {
         return onError(_error, null);
       }
       return onError(_error);
     });
   }
   Stream asStream() =>
       (new StreamController()..addError(_error)..close()).stream;
   Future timeout(Duration duration, {onTimeout}) => this;
 }