message: add proptypes
diff: @@ -41,6 +41,8 @@ export default class EditHistoryMessage extends React.PureComponent {
  static propTypes = {
  // the message event being edited
  mxEvent: PropTypes.instanceOf(MatrixEvent).isRequired,
  + previousEdit: PropTypes.instanceOf(MatrixEvent).isRequired,
  + isBaseEvent: PropTypes.bool,
  };
  constructor(props) {
  