Change fromString constructor to parse static method

tests/standalone/io/http_body_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.
 
 import 'dart:io';
 import 'dart:utf';
 
 import 'package:expect/expect.dart';
 
 void testHttpClientResponseBody() {
   void test(String mimeType,
             List<int> content,
             dynamic expectedBody,
             String type,
             [bool shouldFail = false]) {
     HttpServer.bind("127.0.0.1", 0).then((server) {
       server.listen((request) {
         request.listen(
             (_) {},
             onDone: () {
               request.response.headers.contentType =
-                  new ContentType.fromString(mimeType);
+                  ContentType.parse(mimeType);
               request.response.add(content);
               request.response.close();
             });
       });
 
       var client = new HttpClient();
       client.get("127.0.0.1", server.port, "/")
           .then((request) => request.close())
           .then(HttpBodyHandler.processResponse)
           .then((body) {
@@ skipping 85 matching lines @@
             body.response.close();
           }, onError: (error) {
             if (!shouldFail) Expect.fail("Error unexpected");
           });
 
       var client = new HttpClient();
       client.post("127.0.0.1", server.port, "/")
           .then((request) {
             if (mimeType != null) {
               request.headers.contentType =
-                  new ContentType.fromString(mimeType);
+                  ContentType.parse(mimeType);
             }
             request.add(content);
             return request.close();
           })
           .then((response) {
             if (shouldFail) {
               Expect.equals(HttpStatus.BAD_REQUEST, response.statusCode);
             }
             response.fold(null, (x, y) {});
             client.close();
@@ skipping 23 matching lines @@
        "json",
        true);
 
   test(null, "body".codeUnits, "body".codeUnits, "binary");
 }
 
 void main() {
   testHttpClientResponseBody();
   testHttpServerRequestBody();
 }