markReferenced(varName) {
    const blockVars = this.getScope().findBlockScoped(varName);
    if (blockVars.length > 0) {
      return;
    }

    for (const variable of this.getScope().findFunctionScoped(varName)) {
      variable.markHoisted();
    }
  }
Marks variable referenced in current block scope.

- Not valid block var when not declared in current block scope.

@param  {String} varName