int getInt64(int byteOffset, [ Endianness endian = Endianness.BIG_ENDIAN ])

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 263 and 263 - 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]);