Change fromString constructor to parse static method

pkg/oauth2/lib/src/handle_access_token_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 handle_access_token_response;
 
 import 'dart:io';
 import 'dart:json' as JSON;
 import 'dart:uri';
 
@@ skipping 15 matching lines @@
     Uri tokenEndpoint,
     DateTime startTime,
     List<String> scopes) {
   if (response.statusCode != 200) _handleErrorResponse(response, tokenEndpoint);
 
   void validate(bool condition, String message) =>
     _validate(response, tokenEndpoint, condition, message);
 
   var contentType = response.headers['content-type'];
   if (contentType != null) {
-    contentType = new ContentType.fromString(contentType);
+    contentType = ContentType.parse(contentType);
   }
   validate(contentType != null && contentType.value == "application/json",
       'content-type was "$contentType", expected "application/json"');
 
   var parameters;
   try {
     parameters = JSON.parse(response.body);
   } catch (e) {
     // TODO(nweiz): narrow this catch clause once issue 6775 is fixed.
     validate(false, 'invalid JSON');
@@ skipping 49 matching lines @@
     var reason = '';
     if (response.reasonPhrase != null && !response.reasonPhrase.isEmpty) {
       ' ${response.reasonPhrase}';
     }
     throw new FormatException('OAuth request for "$tokenEndpoint" failed '
         'with status ${response.statusCode}$reason.\n\n${response.body}');
   }
 
   var contentType = response.headers['content-type'];
   if (contentType != null) {
-    contentType = new ContentType.fromString(contentType);
+    contentType = ContentType.parse(contentType);
   }
   validate(contentType != null && contentType.value == "application/json",
       'content-type was "$contentType", expected "application/json"');
 
   var parameters;
   try {
     parameters = JSON.parse(response.body);
   } catch (e) {
     // TODO(nweiz): narrow this catch clause once issue 6775 is fixed.
     validate(false, 'invalid JSON');
@@ skipping 19 matching lines @@
 
 void _validate(
     http.Response response,
     Uri tokenEndpoint,
     bool condition,
     String message) {
   if (condition) return;
   throw new FormatException('Invalid OAuth response for "$tokenEndpoint": '
       '$message.\n\n${response.body}');
 }