utf8Stream.flush(callback)
callback<Function>
如果当前没有正在写入，则将当前缓冲区写入文件。如果 minLength 为零或已经在写入，则不执行任何操作。
【Writes the current buffer to the file if a write was not in progress. Do
nothing if minLength is zero or if it is already writing.】
callback <Function>
如果当前没有正在写入，则将当前缓冲区写入文件。如果 minLength 为零或已经在写入，则不执行任何操作。
【Writes the current buffer to the file if a write was not in progress. Do
nothing if minLength is zero or if it is already writing.】