Fix the OAuth2 documentation to use then rather than chain/transform.

pkg/oauth2/lib/oauth2.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.
 
 /// A client library for authenticating with a remote service via OAuth2 on
 /// behalf of a user, and making authorized HTTP requests with the user's OAuth2
 /// credentials.
 ///
 /// OAuth2 allows a client (the program using this library) to access and
 /// manipulate a resource that's owned by a resource owner (the end user) and
@@ skipping 34 matching lines @@
 ///     final secret = "my client secret";
 ///
 ///     // This is a URL on your application's server. The authorization server
 ///     // will redirect the resource owner here once they've authorized the
 ///     // client. The redirection will include the authorization code in the
 ///     // query parameters.
 ///     final redirectUrl = new Uri.fromString(
 ///         "http://my-site.com/oauth2-redirect");
 ///     
 ///     var credentialsFile = new File("~/.myapp/credentials.json");
-///     return credentialsFile.exists().chain((exists) {
+///     return credentialsFile.exists().then((exists) {
 ///       // If the OAuth2 credentials have already been saved from a previous
 ///       // run, we just want to reload them.
 ///       if (exists) {
-///         return credentialsFile.readAsText().transform((json) {
+///         return credentialsFile.readAsText().then((json) {
 ///           var credentials = new oauth2.Credentials.fromJson(json);
 ///           return new oauth2.Client(identifier, secret, credentials);
 ///         });
 ///       }
 ///     
 ///       // If we don't have OAuth2 credentials yet, we need to get the
 ///       // resource owner to authorize us. We're assuming here that we're a
 ///       // command-line application.
 ///       var grant = new oauth2.AuthorizationCodeGrant(
 ///           identifier, secret, authorizationEndpoint, tokenEndpoint);
 ///     
 ///       // Redirect the resource owner to the authorization URL. This will be
 ///       // a URL on the authorization server (authorizationEndpoint with some
 ///       // additional query parameters). Once the resource owner has
 ///       // authorized, they'll be redirected to `redirectUrl` with an
 ///       // authorization code.
 ///       //
 ///       // `redirect` is an imaginary function that redirects the resource
 ///       // owner's browser.
-///       return redirect(grant.getAuthorizationUrl(redirectUrl)).chain((_) {
+///       return redirect(grant.getAuthorizationUrl(redirectUrl)).then((_) {
 ///         // Another imaginary function that listens for a request to
 ///         // `redirectUrl`.
 ///         return listen(redirectUrl);
-///       }).transform((request) {
+///       }).then((request) {
 ///         // Once the user is redirected to `redirectUrl`, pass the query
 ///         // parameters to the AuthorizationCodeGrant. It will validate them
 ///         // and extract the authorization code to create a new Client.
 ///         return grant.handleAuthorizationResponse(request.queryParameters);
 ///       })
-///     }).chain((client) {
+///     }).then((client) {
 ///       // Once you have a Client, you can use it just like any other HTTP
 ///       // client.
 ///       return client.read("http://example.com/protected-resources.txt")
-///           .transform((result) {
+///           .then((result) {
 ///         // Once we're done with the client, save the credentials file. This
 ///         // ensures that if the credentials were automatically refreshed
 ///         // while using the client, the new credentials are available for the
 ///         // next run of the program.
-///         return credentialsFile.open(FileMode.WRITE).chain((file) {
+///         return credentialsFile.open(FileMode.WRITE).then((file) {
 ///           return file.writeString(client.credentials.toJson());
-///         }).chain((file) => file.close()).transform((_) => result);
+///         }).then((file) => file.close()).then((_) => result);
 ///       });
 ///     }).then(print);
 library oauth2;
 
 export 'src/authorization_code_grant.dart';
 export 'src/client.dart';
 export 'src/credentials.dart';
 export 'src/authorization_exception.dart';
 export 'src/expiration_exception.dart';