class Base {

}

function addGet () {
  return (target: any) => {
    const original = target;

    const constructor : any = function (...args) {
      this.get = function () {
        console.log('GET METHOD');
      }
    };
    constructor.prototype = original.prototype;

    return constructor;
  };
}

@addGet()
class ExtendedBase extends Base {

}

(new ExtendedBase()).get(); //Error: TS2339: Property 'get' does not exist on type 'GlbServersResource'.

(new ExtendedBase() as any).get();

<script>
var txt = `var c = class SomeClass {constructor() {this.text = 'blabla'}}`;
eval.call(this, txt);
console.log(new c());
</script>