[Records] Core library updates

[itsjustkevin]

Expected updates:

• Add Record class as superclass of all record types.

• Add abstract == and hashCode on Record with documentation stating how record equality and hashcode is computed.

• Consider: static bool identical(Record, Record) on class Record, doing deep structure equivalence checks.

• Add documentation to identical saying how it works for record types.

• Add documentation to identityHashCode saying how it works for record types (arbitrarily).

• Add documentation to Expando, WeakReference and Finalizer saying that they do not work with record objects.

Add more as needed.

[lrhn]

We could now add divmod to int as effectively:

(int quotient, int remainder) divmod(int divisor) => (this ~/ divisor, this.remainder(divisor));

Backends can optimize this to ensure we only do one low-level divmod operation, like they try to do now for close-by ~/ and .remainder operations, but users can trust the optimization more this way.

(Could also have divremainder for the above, and make divmod use % and a compatible division instead.)

The usual concern would be whether it should be num.divmod and work on all combinations of integers and doubles, and then we'll have to add language specification rules to type int.divmod(int) as (int, int).

[lrhn]

Another possible related addition would be an extension method on Iterable<E> which provides the index along with the value, Iterable<(int, E)> get indexed;. Q.v. #49928.
It would make a lot of the *Indexed extension methods in package:collection unnecessary.

[vsmenon]

Is there any work left here?

[lrhn]

There is significant work left. I've started on it, but is focusing on modifiers right now.

Concrete plans:

• Iterable.indexed extension method
• Some kind of parallel wait on records of futures, say await (future1, future2, future3).wait or await ~(future1, future2, future3).
• Possibly parallelizing iterables and streams too: (iterable1, iterable2, iterable3).zip.

[mit-mit]

I think this was completed in https://dart-review.googlesource.com/c/sdk/+/288903, @lrhn ?

[lrhn]

I don't think we ever got around to documenting identical and equals for records. I'll do that tomorrow. It's DartDoc only, so it should be uncontroversial.

[mit-mit]

Yeah, but we'll have to cherry pick it, so please file a request for that

[lrhn]

The change is cherry-pick requested. Only thing we need to decide if whether we want to do something about dart:mirrors too. (And that's been answered with "no", so I think we're done now.)