Dart style in the on/once methods

[gil0mendes]

We should provide a more Dart style notation of the on() and once() methods. Of course we will maintain the shorter version :)

// subscribing
emitter.on('Success').listen(eventHandler);
emitter.on('Success', eventHandler);
// subscribing only to the first next event
emitter.once('Success').then(eventHandler);
emitter.once('Success', eventHandler);