fallbackAdapter property
HttpAdapter is a bridge between Dio and HttpClient.
Dio implements standard and friendly API for developer. HttpClient is the real object that makes Http requests.
We can use any HttpClients not just "dart:io:HttpClient" to make the HTTP request. All we need is to provide a HttpClientAdapter.
If you want to customize the HttpClientAdapter you should instead use
either IOHttpClientAdapter on dart:io platforms
or BrowserHttpClientAdapter on dart:html platforms.
Implementation
HttpClientAdapter fallbackAdapter;