cascadeTargets property

A cascade's target value, memoised for the duration of that cascade.

SCE81, and the reason option (b) was reachable at all. Dart evaluates a cascade's target ONCE and then runs its sections in order for their side effects. The interpreter drives await by REPLAY, so a section that suspends means the cascade is re-entered — and without these two maps the target would be re-evaluated and every earlier section re-run: sb..write('a')..write(await f()) would write 'a' twice.

Keyed by the cascade node, so each cascade in flight has its own entry. Removed as soon as the cascade completes, which is what lets a cascade inside a loop start fresh on the next iteration rather than skipping every section it ran last time.

KEYED BY IDENTITY, load-bearing in the tom_d4rt_ast twin for the reason doBodiesStarted records: SAstNode overrides == with a structural toJson() comparison, so a plain Map there would treat two identical-looking cascades as one.