Make instances of HeaderValue and ContentType immutable

pkg/http/lib/src/multipart_file.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.
 
 library multipart_file;
 
 import 'dart:async';
 import 'dart:io';
 
 import '../../../../pkg/pathos/lib/path.dart' as path;
@@ skipping 49 matching lines @@
   }
 
   /// Creates a new [MultipartFile] from a string.
   ///
   /// The encoding to use when translating [value] into bytes is taken from
   /// [contentType] if it has a charset set. Otherwise, it defaults to UTF-8.
   /// [contentType] currently defaults to `text/plain; charset=utf-8`, but in
   /// the future may be inferred from [filename].
   factory MultipartFile.fromString(String field, String value,
       {String filename, ContentType contentType}) {
-    contentType = contentType == null ? new ContentType("text", "plain") :
-        // Make a copy of the original contentType so we can modify charset.
-        new ContentType.fromString(contentType.toString());
+    contentType = contentType == null ? new ContentType("text", "plain")
+                                      : contentType;
     var charset = contentType.charset;
     var encoding = encodingForCharset(contentType.charset, Encoding.UTF_8);
-    contentType.charset = encoding.name;
+    // Make a new contentType with ensured charset.
+    contentType = new ContentType(contentType.primaryType,
+                                  contentType.subType,
+                                  charset: encoding.name,
+                                  parameters: contentType.parameters);
 
     return new MultipartFile.fromBytes(field, encodeString(value, encoding),
         filename: filename,
         contentType: contentType);
   }
 
   // TODO(nweiz): Infer the content-type from the filename.
   /// Creates a new [MultipartFile] from a path to a file on disk.
   ///
   /// [filename] defaults to the basename of [filePath]. [contentType] currently
@@ skipping 15 matching lines @@
   // [MultipartRequest]. This returns a [ByteStream] that should emit the body
   // of the file. The stream may be closed to indicate an empty file.
   ByteStream finalize() {
     if (isFinalized) {
       throw new StateError("Can't finalize a finalized MultipartFile.");
     }
     _isFinalized = true;
     return _stream;
   }
 }