Fix Dart 1.17 strong-mode warnings.

test/utils.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.
 
 /// Helper utilities for testing.
 import "dart:async";
 
 import "package:async/async.dart";
 import "package:test/test.dart";
 
 /// A zero-millisecond timer should wait until after all microtasks.
 Future flushMicrotasks() => new Future.delayed(Duration.ZERO);
 
+typedef void OptionalArgAction([a, b]);
+
 /// A generic unreachable callback function.
 ///
 /// Returns a function that fails the test if it is ever called.
-unreachable(String name) => ([a, b]) => fail("Unreachable: $name");
+OptionalArgAction unreachable(String name) =>
+        ([a, b]) => fail("Unreachable: $name");
 
 // TODO(nweiz): Use the version of this in test when test#418 is fixed.
 /// A matcher that runs a callback in its own zone and asserts that that zone
 /// emits an error that matches [matcher].
 Matcher throwsZoned(matcher) => predicate((callback) {
   var firstError = true;
   runZoned(callback, onError: expectAsync((error, stackTrace) {
     if (firstError) {
       expect(error, matcher);
       firstError = false;
@@ skipping 71 matching lines @@
     stream.listen(add, onError: addError, onDone: completer.complete);
     return completer.future;
   }
 
   Future close() {
     _isClosed = true;
     _doneCompleter.complete(new Future.microtask(_onDone));
     return done;
   }
 }