getFloat64 method Null safety
Returns the floating point number represented by the eight bytes at
byteOffset in this object, in IEEE 754
double-precision binary floating-point format (binary64).
byteOffset must be non-negative, and
byteOffset + 8 must be less than or equal to the length of this object.
double getFloat64(int byteOffset, [Endian endian = Endian.big]);