GeneratorCancelledSignal class

SCF4: the unwind that ends an async* body whose subscription was cancelled.

Dart finishes a cancelled generator as if by return at the pending yield: its finally blocks run, its catch clauses do not, and nothing after the yield executes. The state machine has one route that already behaves that way — the SCC31 "no clause may claim it" route — so this rides it: InterpreterVisitor.selectCatchClause and the synchronous try path decline to match it, finally still runs, and the generator's stream drops it rather than reporting it, since nobody is listening.

Not a RuntimeD4rtException: nothing may treat it as an ordinary runtime condition and try the next strategy, which is exactly what that type is caught for.