Fix ByteBuffer documentation

sdk/lib/io/bytes_builder.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;
 
 /**
  * Builds a list of bytes, allowing bytes and lists of bytes to be added at the
  * end.
  *
@@ skipping 80 matching lines @@
    * The number of bytes in the builder.
    */
   int get length => _length;
 
   /**
    * Returns `true` if the buffer is empty.
    */
   bool get isEmpty => _length == 0;
 
   /**
-   * Returns `true` if the buffer is empty.
+   * Returns `true` if the buffer is not empty.
    */
   bool get isNotEmpty => _length != 0;
 
   /**
    * Clear the contents of the builder.
    */
   void clear() {
     _length = 0;
     _buffer = null;
   }
 
   int _pow2roundup(int x) {
     --x;
     x |= x >> 1;
     x |= x >> 2;
     x |= x >> 4;
     x |= x >> 8;
     x |= x >> 16;
     return x + 1;
   }
 }