Converts a given bit index to the corresponding flag value.
Parameters
bitIndex: number
A bit index.
Returns number
The flag value for the given bit index.
Remarks
In this context, a flag is a 2^n value.
Because the value is returned as a 64 bits floating point number with a 52 bits mantissa,
the given bit index must be less than 53.
Converts a given bit index to the corresponding flag value.