Update the api-docs to mention that DeferredLibrary is deprecated.

sdk/lib/async/deferred_load.dart

 // Copyright (c) 2013, 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.
 
 part of dart.async;
 
 /**
  * Indicates that loading of [libraryName] is deferred.
  *
- * Applies to library imports, when used as metadata.
+ * This class is deprecated. Instead use the syntax:
+ * `import "library.dart" deferred as prefix;`
  *
- * Example usage:
- *
- *     @lazy
- *     import 'foo.dart' as foo;
- *
- *     const lazy = const DeferredLibrary('com.example.foo');
- *
- *     void main() {
- *       foo.method(); // Throws a NoSuchMethodError, foo is not loaded yet.
- *       lazy.load().then(onFooLoaded);
- *     }
- *
- *     void onFooLoaded(_) {
- *       foo.method();
- *     }
+ * Read more about the new syntax
+ * [here](https://www.dartlang.org/docs/spec/deferred-loading.html).
  */
 class DeferredLibrary {
   final String libraryName;
   final String uri;
 
   const DeferredLibrary(this.libraryName, {this.uri});
 
   /**
    * Ensure that [libraryName] has been loaded.
    *
    * If the library fails to load, the Future will complete with a
    * DeferredLoadException.
    */
   external Future<Null> load();
 }
 
 /**
  * Thrown when a deferred library fails to load.
  */
 class DeferredLoadException implements Exception {
   DeferredLoadException(String this._s);
   String toString() => "DeferredLoadException: '$_s'";
   final String _s;
 }