calculateHash function
Calculates the sha256 hash of a file's content.
This is likely to be an expensive operation if the file is large.
You can use this method to check if a file has changes since the last time you took the file's hash.
Throws FileNotFoundException if path
doesn't exist.
Throws NotAFileException if path is
not a file.
Implementation
Digest calculateHash(String path) => waitForEx(
// ignore: discarded_futures
core.calculateHash(path));