Clean up dart:io exceptions.

sdk/lib/io/mime_multipart_parser.dart

 // Copyright (c) 2012, 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 Mime Multipart class representing each part parsed by
  * [MimeMultipartTransformer]. The data is streamed in as it become available.
@@ skipping 380 matching lines @@
   }
 
   void _expectWS(int byte) {
     if (byte != _CharCode.SP && byte != _CharCode.HT) {
       throw new MimeMultipartException("Failed to parse multipart mime 2");
     }
   }
 }
 
 
-class MimeMultipartException implements Exception {
+class MimeMultipartException implements IOException {
   const MimeMultipartException([String this.message = ""]);
   String toString() => "MimeMultipartException: $message";
   final String message;
 }