void writeAsStringSync(String contents, { FileMode mode: FileMode.WRITE, Encoding encoding: UTF8, bool flush: false })

Synchronously write a string to a file.

Opens the file, writes the string in the given encoding, and closes the file.

By default writeAsStringSync creates the file for writing and truncates the file if it already exists. In order to append the bytes to an existing file, pass FileMode.APPEND as the optional mode parameter.

If the flush argument is set to true data written will be flushed to the file system before returning.

Throws a FileSystemException if the operation fails.

Source 

void writeAsStringSync(String contents,
                       {FileMode mode: FileMode.WRITE,
                        Encoding encoding: UTF8,
                        bool flush: false});