put abstract method
Add a key, value pair to the underlying store, overwriting any
existing values. Returns the old value if one existed, otherwise
returns the new value.
put(utf8.encode('foo'), utf8.encode('bar')); // returns 'bar' (new)
put(utf8.encode('foo'), utf8.encode('baz')); // returns 'bar' (old)
get(utf8.encode('foo')); // returns 'baz'
Implementation
Uint8List? put(Uint8List key, Uint8List value);