Use reflection to invoke callbacks in mixins that use metadata as annotation.

gistfile1.dart

@MirrorsUsed(symbols: '', override: '*')
import 'dart:mirrors';

class ReflectionHelper {
  static callMethodsByAnnotation(var object, var symbol) {
    var mirror = reflectClass(object.runtimeType);
    mirror.instanceMembers.forEach((name, method) {
      for (var metadata in method.metadata)
        if (metadata.hasReflectee && metadata.reflectee == symbol)
          reflect(object).invoke(name, []);
    });
  }
}

class Widget {
  static const postPaint = #postPaint;
  void paint() {
    print('Widget.paint()');
    ReflectionHelper.callMethodsByAnnotation(this, postPaint);
  }
}

abstract class BorderMixin {
  @Widget.postPaint
  void paintBorder() {
    print('BorderMixin.paintBorder()');
  }
}

class TextWidget extends Widget with BorderMixin {
  // ...
  @override
  void paint() {
    // ...
    print('TextWidget.paint()');
    // ...
    super.paint();
  }
}

void main() {
  new TextWidget().paint();
}