Fixing many link syntaxes in doc comments

sdk/lib/async/async.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Support for asynchronous programming,
  * with classes such as Future and Stream.
  *
  * Understanding [Future]s and [Stream]s is a prerequisite for
  * writing just about any Dart program.
@@ skipping 49 matching lines @@
  * to convert the sequence of bytes into a sequence
  * of Dart strings.
  *
  * Another common use of streams is for user-generated events
  * in a web app: The following code listens for mouse clicks on a button.
  *
  *     querySelector('#myButton').onClick.listen((_) => print('Click.'));
  *
  * ## Other resources
  *
- * * The [dart:async section of the library tour]
- * (https://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html#ch03-asynchronous-programming):
- * A brief overview of asynchronous programming.
+ * * The [dart:async section of the library tour][asynchronous-programming]:
+ *   A brief overview of asynchronous programming.
  *
- * * [Use Future-Based APIs]
- * (https://www.dartlang.org/docs/tutorials/futures/): A closer look at
- * Futures and how to use them to write asynchronous Dart code.
+ * * [Use Future-Based APIs][futures-tutorial]: A closer look at Futures and
+ *   how to use them to write asynchronous Dart code.
  *
- * * [Futures and Error Handling]
- * (https://www.dartlang.org/articles/futures-and-error-handling/): Everything
- * you wanted to know about handling errors and exceptions when working with
- * Futures (but were afraid to ask).
+ * * [Futures and Error Handling][futures-error-handling]: Everything you
+ *   wanted to know about handling errors and exceptions when working with
+ *   Futures (but were afraid to ask).
  *
  * * [The Event Loop and Dart](https://www.dartlang.org/articles/event-loop/):
- * Learn how Dart handles the event queue and microtask queue, so you can write
- * better asynchronous code with fewer surprises.
+ *   Learn how Dart handles the event queue and microtask queue, so you can
+ *   write better asynchronous code with fewer surprises.
  *
- * * [Asynchronous Unit Testing with Dart]
- * (https://www.dartlang.org/articles/dart-unit-tests/#asynchronous-tests): How
- * to test asynchronous code.
+ * * [test package: Asynchronous Tests][test-readme]: How to test asynchronous
+ *   code.
+ *
+ * [asynchronous-programming]: https://www.dartlang.org/docs/dart-up-and-running/ch03.html#dartasync---asynchronous-programming
+ * [futures-tutorial]: https://www.dartlang.org/docs/tutorials/futures/
+ * [futures-error-handling]: https://www.dartlang.org/articles/futures-and-error-handling/
+ * [test-readme]: https://pub.dartlang.org/packages/test
  */
 library dart.async;
 
 import "dart:collection";
 import "dart:_internal" show printToZone, printToConsole,
                              IterableElementError;
 
 part 'async_error.dart';
 part 'broadcast_stream_controller.dart';
 part 'deferred_load.dart';
 part 'future.dart';
 part 'future_impl.dart';
 part 'schedule_microtask.dart';
 part 'stream.dart';
 part 'stream_controller.dart';
 part 'stream_impl.dart';
 part 'stream_pipe.dart';
 part 'stream_transformers.dart';
 part 'timer.dart';
 part 'zone.dart';