message: moved func decl to top leve and typed it
diff: import {Path, TransformExitResult} from '@romejs/js-compiler';
  import {descriptions} from '@romejs/diagnostics';
  +function isQuantifiedMinZero(el: AnyRegExpBodyItem): bool {
  + return el.type === 'RegExpQuantified' && el.min == 0;
  +}
  +
  export default {
  name: 'noEmptyCharacterClass',
  enter(path: Path): TransformExitResult {
  const {context, node} = path;
  if (node.type === 'RegExpSubExpression') {
  - let isQuantifiedMinZero = el => el.type === 'RegExpQuantified' && el.min == 0;
  if (node.body.every(isQuantifiedMinZero)) {
  context.addNodeDiagnostic(node, descriptions.LINT.EMPTY_MATCHES);
  }
  