Modernize the package's style.

lib/src/null_stream_sink.dart

 // Copyright (c) 2016, 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.
 
-library async.null_stream_sink;
-
 import 'dart:async';
 
 /// A [StreamSink] that discards all events.
 ///
 /// The sink silently drops events until [close] is called, at which point it
 /// throws [StateError]s when events are added. This is the same behavior as a
 /// sink whose remote end has closed, such as when a [WebSocket] connection has
 /// been closed.
 ///
 /// This can be used when a sink is needed but no events are actually intended
@@ skipping 64 matching lines @@
     if (_addingStream) {
       throw new StateError("Cannot add to a sink while adding a stream.");
     }
   }
 
   Future close() {
     _closed = true;
     return done;
   }
 }