const Goa = imports.gi.Goa;

let client = Goa.Client.new_sync(null);
let accounts = client.get_accounts();
for (let i in accounts) {
    //Now account is a instance of org.gnome.OnlineAccounts.Account
    //DBus interface (GoaAccountProxy instance)
    let account = accounts[i].get_account();

    //Getting a property does not work since it wasn't injected with
    //the convenience DBus binding from Gio.js.
    //print(account.ProviderName);

    //I need to do something like
    print(account.get_cached_property("ProviderName").deep_unpack());
}