DeferredScreen class

Wraps a screen whose Dart library is loaded on demand via import '...' deferred as alias; + alias.loadLibrary().

On the first navigation to the route, dart2js fetches the deferred chunk and resolves the future returned by loader. Subsequent navigations skip the fetch — loadLibrary() is idempotent and returns immediately. Once a loader has resolved, DeferredScreen remembers that fact in a static set keyed by the loader tear-off (top-level loadLibrary tear-offs are canonical, so identity comparison is stable). On the next mount it short-circuits the FutureBuilder and renders builder synchronously on the very first frame, eliminating the one-frame ConnectionState.waiting flash that would otherwise show on every revisit.

While a chunk is genuinely fetching, the placeholder spinner fades in with a small delay so sub-300 ms loads pass invisibly — the user sees the previous screen until the new screen is ready.

Native (iOS/Android) builds still have to call loadLibrary() once to mark the library "loaded" — accessing a deferred symbol before that throws _DeferredNotLoadedError. The completion runs via a microtask, so widget tests that mount a route through this widget must await tester.pumpAndSettle() (or pump for a non-zero duration) before asserting on the loaded screen.