operator []= method
Set the value at the given index.
The value should be in the range 0 to 255. An integer, which is not in
that range, is converted to a byte as if by value.toUnsigned(8).
Implementation
void operator []=(int index, int value) {
IndexError.check(index, length, indexable: this);
mutableBytes.cast<Uint8>()[index] = value;
}