void setInt64(int byteOffset, int value, [ Endianness endian = Endianness.BIG_ENDIAN ])

Sets the eight bytes starting at the specified byteOffset in this object to the two's complement binary representation of the specified value, which must fit in eight bytes.

In other words, value must lie between -263 and 263 - 1, inclusive.

Throws RangeError if byteOffset is negative, or byteOffset + 8 is greater than the length of this object.

Source 

void setInt64(int byteOffset, int value,
    [Endianness endian = Endianness.BIG_ENDIAN]);