class Null should be declared explicitly in the core library

[gbracha]

Null is a real class and should have a declaration just like Object or anything else.

[munificent]

With Futures, you often have a future that completes to a side-effect and no meaningful value. In that case, it always completes to null. If we have a public Null class, should we start using Future<Null> to indicate that?

[lrhn]

In Dart2JS it already has.
Is this requesting that the class be public.

[gbracha]

Re comment 2: yes, the class is named Null, so it cannot be private.

[rmacnak-google]

Marked this as being blocked by #12364.

[lrhn]

Added Fixed label.