addSingleton<T>  method 
      
  
void
addSingleton<T>(})
        
    
inherited
    Register a Singleton instance.
It will generate a single instance for the duration of
the application, or until manually removed.
The object will be started as soon as it is registered.
injector.addSingleton(MyController.new);
When [key] is provided this instance only can be found by key
Implementation
@override
void addSingleton<T>(
  Function constructor, {
  BindConfig<T>? config,
  String? key,
}) {
  _add<T>(
    constructor,
    type: BindType.singleton,
    config: config,
    key: key,
  );
}