Fix Dart 1.17 strong-mode warnings.

test/subscription_stream_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" show SubscriptionStream;
 import "package:test/test.dart";
 
 import "utils.dart";
 
 main() {
   test("subscription stream of an entire subscription", () async {
     var stream = createStream();
     var subscription = stream.listen(null);
     var subscriptionStream = new SubscriptionStream<int>(subscription);
     await flushMicrotasks();
     expect(subscriptionStream.toList(), completion([1, 2, 3, 4]));
   });
 
   test("subscription stream after two events", () async {
     var stream = createStream();
     var skips = 0;
     var completer = new Completer();
-    var subscription;
+    StreamSubscription<int> subscription;
     subscription = stream.listen((value) {
       ++skips;
       expect(value, skips);
       if (skips == 2) {
         completer.complete(new SubscriptionStream<int>(subscription));
       }
     });
     var subscriptionStream = await completer.future;
     await flushMicrotasks();
     expect(subscriptionStream.toList(), completion([3, 4]));
@@ skipping 125 matching lines @@
   yield 4;
 }
 
 Stream<int> createErrorStream([Completer onCancel]) async* {
   bool canceled = true;
   try {
     yield 1;
     await flushMicrotasks();
     yield 2;
     await flushMicrotasks();
-    yield* new Future.error("To err is divine!").asStream();
+    yield* new Future<int>.error("To err is divine!").asStream();
     await flushMicrotasks();
     yield 4;
     await flushMicrotasks();
     canceled = false;
   } finally {
     // Completes before the "done", but should be after all events.
     if (canceled && onCancel != null) {
       await flushMicrotasks();
       onCancel.complete();
     }
   }
 }
 
 Stream<int> createLongStream() async* {
   for (int i = 0; i < 200; i++) yield i;
 }