Remove deprecated Strings class.

pkg/oauth2/lib/src/credentials.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 credentials;
 
 import 'dart:async';
 import 'dart:json' as JSON;
 import 'dart:uri';
 
@@ skipping 153 matching lines @@
 
       var fields = {
         "grant_type": "refresh_token",
         "refresh_token": refreshToken,
         // 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": identifier,
         "client_secret": secret
       };
-      if (!scopes.isEmpty) fields["scope"] = Strings.join(scopes, ' ');
+      if (!scopes.isEmpty) fields["scope"] = scopes.join(' ');
 
       return httpClient.post(tokenEndpoint, fields: fields);
     }).then((response) {
       return handleAccessTokenResponse(
           response, tokenEndpoint, startTime, scopes);
     }).then((credentials) {
       // The authorization server may issue a new refresh token. If it doesn't,
       // we should re-use the one we already have.
       if (credentials.refreshToken != null) return credentials;
       return new Credentials(
           credentials.accessToken,
           this.refreshToken,
           credentials.tokenEndpoint,
           credentials.scopes,
           credentials.expiration);
     });
   }
 }