handleUpdate (type, target) {
    logger.debug(type, target)
    if (!path.isAbsolute(target)) {
      target = path.join(this.context.sourceDir, target)
    }
    if (target.endsWith('.js')) {
      // Bust cache.
      delete require.cache[target]
    }
    this.emit('fileChanged', {
      type,
      target
    })
  }
Hande file's update, need to re-prepare app context.

@param {string} type
@param {string} target