catchEnvironments property

The child environment each catch clause's body runs in, by clause.

SCE79. _handleAsyncError used to define the exception variable in the FUNCTION's environment — its own comment said "can cause collisions" — so in an async function the variable outlived its block and, worse, silently overwrote a caller's local of the same name. e is one of the most common names there is, and nothing threw or logged: the value was simply wrong from that point on. visitTryStatement has always created a child environment, which is what Dart requires.

WHY A MAP AND NOT A STACK, which is the design question this posed. A stack has to be POPPED, and the state machine resumes at a node rather than executing a block — so every exit from a catch block (fallthrough, return, rethrow, break, an error) would need to pop, and one missed exit leaks the scope again in a way nothing would notice. Keyed by clause, the binding is SELECTED structurally instead: a node lexically inside the clause gets the environment, a node outside it never asks. There is no exit to instrument because there is nothing to undo.

A re-entered clause overwrites its entry, so a try inside a loop gets a fresh environment per iteration rather than resuming a stale one.

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