Change fromString constructor to parse static method

pkg/http/lib/src/response.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 response;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:typed_data';
 
@@ skipping 71 matching lines @@
 /// Returns the encoding to use for a response with the given headers. This
 /// defaults to [Encoding.ISO_8859_1] if the headers don't specify a charset or
 /// if that charset is unknown.
 Encoding _encodingForHeaders(Map<String, String> headers) =>
   encodingForCharset(_contentTypeForHeaders(headers).charset);
 
 /// Returns the [ContentType] object for the given headers. Defaults to
 /// `application/octet-stream`.
 ContentType _contentTypeForHeaders(Map<String, String> headers) {
   var contentType = headers[HttpHeaders.CONTENT_TYPE];
-  if (contentType != null) return new ContentType.fromString(contentType);
+  if (contentType != null) return ContentType.parse(contentType);
   return new ContentType("application", "octet-stream");
 }