Returns the (possibly negative) integer represented by the eight bytes at
the specified byteOffset in this object, in two's complement binary
form.
The return value will be between 2<sup>63</sup> and 2<sup>63</sup> - 1, inclusive.
Throws RangeError if byteOffset is negative, or
byteOffset + 8 is greater than the length of this object.
Source
int getInt64(int byteOffset, [Endianness endian = Endianness.BIG_ENDIAN]);