Add utility methods for sending non-form data in pkg/http.

pkg/oauth2/lib/src/authorization_code_grant.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 authorization_code_grant;
 
 import 'dart:async';
 
 import 'package:http/http.dart' as http;
 
@@ skipping 210 matching lines @@
       _state = _FINISHED_STATE;
 
       return _handleAuthorizationCode(authorizationCode);
     });
   }
 
   /// This works just like [handleAuthorizationCode], except it doesn't validate
   /// the state beforehand.
   Future<Client> _handleAuthorizationCode(String authorizationCode) {
     var startTime = new DateTime.now();
-    return _httpClient.post(this.tokenEndpoint, fields: {
+    return _httpClient.post(this.tokenEndpoint, body: {
       "grant_type": "authorization_code",
       "code": authorizationCode,
       "redirect_uri": this._redirectEndpoint.toString(),
       // TODO(nweiz): the spec recommends that HTTP basic auth be used in
       // preference to form parameters, but Google doesn't support that. Should
       // it be configurable?
       "client_id": this.identifier,
       "client_secret": this.secret
     }).then((response) {
       var credentials = handleAccessTokenResponse(
           response, tokenEndpoint, startTime, _scopes);
       return new Client(
           this.identifier, this.secret, credentials, httpClient: _httpClient);
     });
   }
 
   /// Closes the grant and frees its resources.
   ///
   /// This will close the underlying HTTP client, which is shared by the
   /// [Client] created by this grant, so it's not safe to close the grant and
   /// continue using the client.
   void close() {
     if (_httpClient != null) _httpClient.close();
     _httpClient = null;
   }
 }