double getFloat64(int byteOffset, [ Endianness endian = Endianness.BIG_ENDIAN ])

Returns the floating point number represented by the eight bytes at the specified byteOffset in this object, in IEEE 754 double-precision binary floating-point format (binary64).

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

Source 

double getFloat64(int byteOffset,
                  [Endianness endian = Endianness.BIG_ENDIAN]);