Remove deprecated Strings class.

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 'dart:uri';
 
 // TODO(nweiz): This should be a "package:" import. See issue 6745.
@@ skipping 121 matching lines @@
     this._redirectEndpoint = redirect;
     this._scopes = scopes;
     this._stateString = state;
     var parameters = {
       "response_type": "code",
       "client_id": this.identifier,
       "redirect_uri": redirect.toString()
     };
 
     if (state != null) parameters['state'] = state;
-    if (!scopes.isEmpty) parameters['scope'] = Strings.join(scopes, ' ');
+    if (!scopes.isEmpty) parameters['scope'] = scopes.join(' ');
 
     return addQueryParameters(this.authorizationEndpoint, parameters);
   }
 
   /// Processes the query parameters added to a redirect from the authorization
   /// server. Note that this "response" is not an HTTP response, but rather the
   /// data passed to a server controlled by the client as query parameters on
   /// the redirect URL.
   ///
   /// It is a [StateError] to call this more than once, to call it before
@@ skipping 97 matching lines @@
   /// 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;
   }
 }