Detect impossible `is`es

[lishaduck]

Dart detects when an equals will never evaluate to true because the types aren't overlapping. It'd be nice if I could also get a warning for non-overlapping types in an is check, where the value on the left will never be of the type on right.

That feels overly short. Anything else you need?

[FMorschel]

I think this may be a dupe of #59503.

[lrhn]

It's very few types where it's absolutely guaranteed that a value of one static type cannot be an instance of another type.

There are some (a record type cannot be a non-record type or a record-type with a different shape, a function type cannot be a non-function type, a non-nullable type can't be Null, or vice-versa, and recursively on covariant part-types).

For interface or extension types, it's much harder to say that one type cannot also implement another. That's where most uses of is are. An int can't be a String, but that's about it. It requires internal knowledge about the implementation to make that judgement. (One could easily assumethat anintcouldn't be adouble`, but JS compiled integers are doubles.)

And also a duplicate of #59503.