The fe5b75d92 merge took upstream's widget_async.rs and dropped the
wm isolate-entry API (IsolateEntry, enter_isolate, leave_isolate)
while lib.rs and apps/flow-ui + apps/wm still use it, breaking
makepad-widgets. Restored verbatim from 77d91385f; structs unchanged
so the code applies as-is. makepad-widgets, makepad-app-flow-ui and
makepad-wm all check clean.