Make instances of HeaderValue and ContentType immutable

tests/standalone/io/http_advanced_test.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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import 'dart:async';
@@ skipping 329 matching lines @@
             if (responses == 2) {
               httpClient.close();
               server.shutdown();
               completer.complete(true);
             }
           });
     }
 
     httpClient.get("127.0.0.1", port, "/contenttype1")
         .then((request) {
-          ContentType contentType = new ContentType();
-          contentType.value = "text/html";
-          contentType.parameters["charset"] = "utf-8";
-          request.headers.contentType = contentType;
+          request.headers.contentType =
+              new ContentType("text", "html", "utf-8");
           return request.close();
         })
         .then(processResponse);
 
     httpClient.get("127.0.0.1", port, "/contenttype2")
         .then((request) {
           request.headers.set(HttpHeaders.CONTENT_TYPE,
                               "text/html;  charset = utf-8");
           return request.close();
         })
@@ skipping 59 matching lines @@
 
 void main() {
   testHost().then((_) {
     return testExpires().then((_) {
       return testContentType().then((_) {
         return testCookies();
       });
     });
   });
 }