Warn when adding something to a closed sink and improve documentation

sdk/lib/io/io_sink.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.io;
 
 /**
  * A combined byte and text output.
  *
  * An [IOSink] combines a [StreamSink] of bytes with a [StringSink],
@@ skipping 130 matching lines @@
   final Completer _doneCompleter = new Completer();
   StreamController<T> _controllerInstance;
   Completer _controllerCompleter;
   bool _isClosed = false;
   bool _isBound = false;
   bool _hasError = false;
 
   _StreamSinkImpl(this._target);
 
   void add(T data) {
-    if (_isClosed) return;
+    if (_isClosed) throw new StateError("StreamSink is closed");
     _controller.add(data);
   }
 
   void addError(error, [StackTrace stackTrace]) {
+    if (_isClosed) throw new StateError("StreamSink is closed");
     _controller.addError(error, stackTrace);
   }
 
   Future addStream(Stream<T> stream) {
     if (_isBound) {
       throw new StateError("StreamSink is already bound to a stream");
     }
     _isBound = true;
     if (_hasError) return done;
     // Wait for any sync operations to complete.
@@ skipping 134 matching lines @@
 
   void writeln([Object object = ""]) {
     write(object);
     write("\n");
   }
 
   void writeCharCode(int charCode) {
     write(new String.fromCharCode(charCode));
   }
 }