Fix Dart 1.17 strong-mode warnings.

test/stream_splitter_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:async/async.dart';
 import 'package:test/test.dart';
 
 main() {
-  var controller;
+  StreamController<int> controller;
   var splitter;
   setUp(() {
     controller = new StreamController<int>();
     splitter = new StreamSplitter<int>(controller.stream);
   });
 
   test("a branch that's created before the stream starts to replay it",
       () async {
     var events = [];
     var branch = splitter.split();
@@ skipping 255 matching lines @@
     expect(branches[0].toList(), completion(equals([1, 2, 3])));
     expect(branches[1].toList(), completion(equals([1, 2, 3])));
     expect(branches[2].toList(), completion(equals([1, 2, 3])));
     expect(branches[3].toList(), completion(equals([1, 2, 3])));
     expect(branches[4].toList(), completion(equals([1, 2, 3])));
   });
 }
 
 /// Wait for all microtasks to complete.
 Future flushMicrotasks() => new Future.delayed(Duration.ZERO);