let editableThings = new WeakMap();
function finisherFor(prop) {
  function finisher(C) {
    let list = editableThings.get(C);
    if (!list) {
      list = [];
      editableThings.set(C, list);
    }

    list.push(prop);
  }
}

function Editable() {
  return function ({ key, ... props}) {
    return { key, ... props, finisher: finisherFor(key) };
  }
}

function getEditableProps(C) {
  return editableThings.get(C) || [];
}


class Blog {
  @Editable title;
  @Editable author;
}


print(getEditableProps(Blog)); // ['title', 'author']