Returns the (possibly negative) integer represented by the eight bytes at
byteOffset in this object, in two's complement binary
The return value will be between -263 and 263 - 1, inclusive.
Throws RangeError if
byteOffset is negative, or
byteOffset + 8 is greater than the length of this object.
int getInt64(int byteOffset, [Endian endian = Endian.big]);