void setFloat64(int byteOffset, double value, [Endianness endian = Endianness.BIG_ENDIAN])

Sets the eight bytes starting at the specified byteOffset in this object to the IEEE 754 double-precision binary floating-point (binary64) representation of the specified value.

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

Source 

void setFloat64(int byteOffset,
                double value,
                [Endianness endian = Endianness.BIG_ENDIAN]);