Dynamically Add Method to Objects in Dart

Use noSuchMethod()

class Widget {
  final methods = <Symbol, Function>{};

  noSuchMethod(Invocation i) {
    if (i.isMethod && methods.containsKey(i.memberName)) {
      return Function.apply(methods[i.memberName],
          i.positionalArguments, i.namedArguments);
    return super.noSuchMethod(i);

Create Widget instance and assign a function to methods mapping with a custom name, i.e. welcome

var widget = Widget();
widget.methods[#welcome] = () => print('Hello, Dart');

Invoke the welcome method on the Widget instance as you'd have done with regular methods