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]);