readInto abstract method
Reads bytes into an existing buffer.
Reads bytes and writes then into the range of buffer
from start to end.
The start must be non-negative and no greater than buffer.length.
If end is omitted, it defaults to buffer.length.
Otherwise end must be no less than start
and no greater than buffer.length.
Returns the number of bytes read. This maybe be less than end - start
if the file doesn't have that many bytes to read.
Implementation
Future<int> readInto(List<int> buffer, [int start = 0, int? end]);