Reads into an existing List from the file. If start is present,
the bytes will be filled into buffer from at index start, otherwise
index 0. If end is present, the end - start bytes will be read into
buffer, otherwise up to buffer.length. If end == start nothing
happends.
Returns a Future<int> that completes with the number of bytes read.
Source
Future<int> readInto(List<int> buffer, [int start = 0, int end]);