toInt64 method
Sign-extending widen, then reinterpret — e.g. Int32.minusOne
widens to all-ones, so toUint64() is Uint64.max, not Uint32.max.
Implementation
Int64 toInt64() => Int64.unsafe(
Uint64.unsafe((_bits & _signBit32) != 0 ? BinaryOps.mask32 : 0, _bits),
);