Add missing copyright header (dart-lang/stream_transform#70)

pkgs/stream_transform/test/combine_latest_all_test.dart

diff --git a/pkgs/stream_transform/test/combine_latest_all_test.dart b/pkgs/stream_transform/test/combine_latest_all_test.dart
index 9cf3a91..cdfedb0 100644
--- a/pkgs/stream_transform/test/combine_latest_all_test.dart
+++ b/pkgs/stream_transform/test/combine_latest_all_test.dart
@@ -1,5 +1,11 @@
+// Copyright (c) 2019, 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:test/test.dart';
+
 import 'package:stream_transform/stream_transform.dart';
 
 Future<void> tick() => Future(() {});