Warn when adding something to a closed sink and improve documentation

sdk/lib/core/sink.dart

 // Copyright (c) 2014, 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.core;
 
 /**
  * A generic destination for data.
  *
  * Multiple data values can be put into a sink, and when no more data is
  * available, the sink should be closed.
  *
  * This is a generic interface that other data receivers can implement.
  */
 abstract class Sink<T> {
   /**
-   * Put the data into the sink.
+   * Puts the [data] into the sink.

[Lasse Reichstein Nielsen, 2017/04/26 08:26:19]

Change Puts-into into Adds-to. Or maybe "Pass-to"

It's a silly documentation anyway - it just repeats the method name, but I guess
that's the cost of being completely generic.

[floitsch, 2017/05/01 16:46:25]

I'm ok with repeating the name, if there is more dartdoc.
done.

    *
    * Must not be called after a call to [close].
    */
   void add(T data);
 
   /**
-   * Tell the sink that no further data will be added.
+   * Closes the sink.
+   *
+   * The [add] method must not be called after this method.
    *
    * Calling this method more than once is allowed, but does nothing.
-   *
-   * The [add] method must not be called after this method.
    */
   void close();
 }