create abstract method
Creates the file.
Returns a Future<File> that completes with
the file when it has been created.
If recursive is false, the default, the file is created only if
all directories in its path already exist. If recursive is true, any
non-existing parent paths are created first.
If exclusive is true and to-be-created file already exists, this
operation completes the future with a PathExistsException.
If exclusive is false, existing files are left untouched by create.
Calling create on an existing file still might fail if there are
restrictive permissions on the file.
Completes the future with a FileSystemException if the operation fails.
Implementation
Future<File> create({bool recursive = false, bool exclusive = false});