ZoneSpecification should be extendable

[nex3]

It would be very useful to be able to extend ZoneSpecification and override its handlers. Calling [new ZoneSpecification] can be useful when relatively few functions are being declared, but for larger specifications it gets very unwieldy.

There are currently two major obstacles to this:

* ZoneSpecification's constructors are both factories.

• ZoneSpecification's handlers are declared as getters, which aren't friendly to subclassing due to issue Methods should be able to override function-typed getters #15108.

[nex3]

I've realized since filing this that it's important for the way zones currently work that some of the fields return null, which makes working around issue #15108 difficult. Feel free to either mark this as blocked on issue #15108, or just consider it a more general issue of "I'd like a nicer way of writing complex zone specifications".

[floitschG]

As designed. The current API is restrictive but this allows us to have some assumptions on the values of the specification (like them being final).

The most annoying approach with the current approach is that it is difficult to have shared state. Either store it in the zone (in the values-map), or use closures that capture the same state.

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Added AsDesigned label.