1. protected
void registerServiceExtension({@required String name, @required ServiceExtensionCallback callback })

Registers a service extension method with the given name (full name "ext.flutter.name"). The given callback is called when the extension method is called. The callback must return a Future that either eventually completes to a return value in the form of a name/value map where the values can all be converted to JSON using JSON.encode, or fails. In case of failure, the failure is reported to the remote caller and is dumped to the logs.

The returned map will be mutated.

Source 

@protected
void registerServiceExtension({
  @required String name,
  @required ServiceExtensionCallback callback
}) {
  assert(name != null);
  assert(callback != null);
  final String methodName = 'ext.flutter.$name';
  developer.registerExtension(methodName, (String method, Map<String, String> parameters) async {
    assert(method == methodName);
    dynamic caughtException;
    StackTrace caughtStack;
    Map<String, dynamic> result;
    try {
      result = await callback(parameters);
    } catch (exception, stack) {
      caughtException = exception;
      caughtStack = stack;
    }
    if (caughtException == null) {
      result['type'] = '_extensionType';
      result['method'] = method;
      return new developer.ServiceExtensionResponse.result(JSON.encode(result));
    } else {
      FlutterError.reportError(new FlutterErrorDetails(
        exception: caughtException,
        stack: caughtStack,
        context: 'during a service extension callback for "$method"'
      ));
      return new developer.ServiceExtensionResponse.error(
        developer.ServiceExtensionResponse.extensionError,
        JSON.encode(<String, dynamic>{
          'exception': caughtException.toString(),
          'stack': caughtStack.toString(),
          'method': method
        })
      );
    }
  });
}