// Make sure to replace <YOUR_LOCAL_IP> with 
// the external IP of your computer if you're using Android. 
// Note that we're using port 8888 which is Charles' default.
String proxy = Platform.isAndroid ? '<YOUR_LOCAL_IP>:8888' : 'localhost:8888';

// Create a new HttpClient instance.
HttpClient httpClient = new HttpClient();

// Hook into the findProxy callback to set
// the client's proxy.
httpClient.findProxy = (uri) {
  return "PROXY $proxy;";
};

// This is a workaround to allow Charles to receive
// SSL payloads when your app is running on Android.
httpClient.badCertificateCallback = 
  ((X509Certificate cert, String host, int port) => Platform.isAndroid);

// Pass your newly instantiated HttpClient to http.IOClient.
IOClient myClient = IOClient(httpClient);

// Make your request as normal.
var response = myClient.get('/my-url');