Migrated test block 168 to Dart 2.0.

tests/lib_strong/async/first_regression_test.dart

-// Copyright (c) 2012, 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.
-
-// Regression test for http://dartbug.com/7964
-
-library first_regression_test;
-
-import 'dart:async';
-import 'package:unittest/unittest.dart';
-
-class DoubleTransformer<T> implements StreamTransformer<T, T> {
-  Stream<T> bind(Stream<T> stream) {
-    var transformer = new StreamTransformer<T, T>.fromHandlers(
-        handleData: (T data, EventSink<T> sink) {
-      sink.add(data);
-      sink.add(data);
-    });
-    return transformer.bind(stream);
-  }
-}
-
-main() {
-  test("Double event before first", () {
-    // This should not crash. Did crash by trying to complete future more
-    // than once.
-    new Stream.fromIterable([1, 2])
-        .transform(new DoubleTransformer())
-        .first
-        .then(expectAsync((e) {}));
-  });
-}