Fine. Add ClassItem.allSubtypes and precise tracking for ClassElementImpl.allSubtypes

We recorded requirements for `ClassElementImpl.allSubtypes` indirectly
by touching `LibraryElement.children`. That over-approximated
dependencies to "any child in the library," making the requirement too
wide and imprecise. Unrelated top-level edits could trigger reanalysis,
and there was no explicit validation of the class’s actual subtype set.

This change introduces explicit and precise tracking of subtypes:

- Add `ClassItem.allSubtypes` to the element manifest and serialize it.
- Populate `allSubtypes` during manifest build
  (`_fillClassAllSubtypes`) from the class’s computed subtypes.
- Record and validate `allSubtypes` in `InterfaceItemRequirements`; a
  mismatch now surfaces via `InterfaceChildrenIdsMismatch`.
- Include `allSubtypes` in the requirements hash so cache keys reflect
  subtype changes.
- Wrap `ClassElementImpl.allSubtypes` with `alreadyRecorded()` and mark
  it `@trackedDirectly` to capture precise requirements.
- Extend result printers to display `allSubtypes`.
- Bump `DATA_VERSION` to 570.

Semantics of `allSubtypes`:
- `final` classes: all subtypes are within the declaring library.
- `sealed` classes: known only if each subtype is `final` or `sealed`;
  otherwise the value is `null`.
- Mixins make the set unknown; enums count as subtypes when applicable.

Impact: tighter and more accurate invalidation—changes to a class’s
subtype set are detected, while unrelated library changes no longer
cause unnecessary reanalysis.

Change-Id: I1d80c80da41feb3f1346026817e06291c0ca1216
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/452845
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>