Fix a broken oauth2 test.

pkg/oauth2/lib/src/client.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 oauth2_client;
 
 import 'dart:async';
 import 'dart:uri';
 
 import 'package:http/http.dart' as http;
@@ skipping 92 matching lines @@
         return response;
       }
 
       if (authenticate.scheme != 'bearer') return response;
 
       var params = authenticate.parameters;
       if (!params.containsKey('error')) return response;
 
       throw new AuthorizationException(
           params['error'], params['error_description'],
-          Uri.parse(params['error_uri']));
+          params['error_uri'] == null ? null : Uri.parse(params['error_uri']));
     });
   }
 
   /// Explicitly refreshes this client's credentials. Returns this client.
   ///
   /// This will throw a [StateError] if the [Credentials] can't be refreshed, an
   /// [AuthorizationException] if refreshing the credentials fails, or a
   /// [FormatError] if the authorization server returns invalid responses.
   ///
   /// You may request different scopes than the default by passing in
@@ skipping 14 matching lines @@
       return this;
     });
   }
 
   /// Closes this client and its underlying HTTP client.
   void close() {
     if (_httpClient != null) _httpClient.close();
     _httpClient = null;
   }
 }