Async-ify.

README.md

 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
 lives on a remote server. The client directs the resource owner to an
 authorization server (usually but not always the same as the server that
 hosts the resource), where the resource owner tells the authorization server
 to give the client an access token. This token serves as proof that the
@@ skipping 30 matching lines @@
 // with certainty that a client is who it claims to be.
 final identifier = "my client identifier";
 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 = Uri.parse("http://my-site.com/oauth2-redirect");
 
-var credentialsFile = new File("~/.myapp/credentials.json");
-return credentialsFile.exists().then((exists) {
-  // If the OAuth2 credentials have already been saved from a previous
-  // run, we just want to reload them.
+/// A file in which the users credentials are stored persistently. If the server
+/// issues a refresh token allowing the client to refresh outdated credentials,
+/// these may be valid indefinitely, meaning the user never has to
+/// re-authenticate.
+final credentialsFile = new File("~/.myapp/credentials.json");
+
+/// Either load an OAuth2 client from saved credentials or authenticate a new
+/// one.
+Future<oauth2.Client> getClient() async {
+  var exists = await credentialsFile.exists();
+
+  // If the OAuth2 credentials have already been saved from a previous run, we
+  // just want to reload them.
   if (exists) {
-    return credentialsFile.readAsString().then((json) {
-      var credentials = new oauth2.Credentials.fromJson(json);
-      return new oauth2.Client(identifier, secret, credentials);
-    });
+    var credentials = new oauth2.Credentials.fromJson(
+        await credentialsFile.readAsString());
+    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.
+  // 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 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)).then((_) {
-    // Another imaginary function that listens for a request to
-    // `redirectUrl`.
-    return listen(redirectUrl);
-  }).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.uri.queryParameters);
-  })
-}).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")
-      .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).then((file) {
-      return file.writeString(client.credentials.toJson());
-    }).then((file) => file.close()).then((_) => result);
-  });
-}).then(print);
+  await redirect(grant.getAuthorizationUrl(redirectUrl));
+  
+  // Another imaginary function that listens for a request to `redirectUrl`.
+  var request = await listen(redirectUrl);
+
+  // 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 await grant.handleAuthorizationResponse(request.uri.queryParameters);
+}
+
+main() async {
+  var client = await loadClient();
+
+  // Once you have a Client, you can use it just like any other HTTP client.
+  var result = client.read("http://example.com/protected-resources.txt");
+
+  // 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.
+  await credentialsFile.writeAsString(client.credentials.toJson());
+
+  print(result);
+}
 ```