pkg/mime: Removed mutable state from MimeMultipartTransformer

pkg/mime/lib/src/mime_shared.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.
+library mime.shared;
+
+import 'dart:async';
+
+class MimeMultipartException implements Exception {
+  final String message;
+
+  const MimeMultipartException([String this.message = ""]);
+
+  String toString() => "MimeMultipartException: $message";
+}
+
+/**
+ * A Mime Multipart class representing each part parsed by
+ * [MimeMultipartTransformer]. The data is streamed in as it become available.
+ */
+abstract class MimeMultipart extends Stream<List<int>> {
+  Map<String, String> get headers;
+}