Consider making Optional not allocate a new absent per type.

[lrhn]

I see code like Optional<String> something = const Optional<String>.absent();. This allocates a new empty optional per type, even though they all behave the same.

Have you considered using a single Optional<Null> for all absent optionals?

Even if you can't change the API at this point, you could still make the above allocate an Optional<Null> instead by writing the constructor as:

const factory Optional.absent() = Optional<Null>._absent;
const Optional._absent() : _value = null;

(A little hack-ish, but it should work :).

Since Optional<Null> is a subtype of any other Optional<X>, you get to reuse the same const instance in all the places it's created without type issues.