Remove warnings in tests, and even fix analyzer hints.

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.
 library async.test.util;
 
 import "dart:async";
+import "package:test/test.dart";
 
 /// A zero-millisecond timer should wait until after all microtasks.
 Future flushMicrotasks() => new Future.delayed(Duration.ZERO);
 
 /// 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");
 
 /// A badly behaved stream which throws if it's ever listened to.
 ///
 /// Can be used to test cases where a stream should not be used.
 class UnusableStream extends Stream {
   listen(onData, {onError, onDone, cancelOnError}) {
     throw new UnimplementedError("Gotcha!");
   }
 }