Change JSBoxedDartObject to JSAny (#51634)

Closes dart-lang/sdk#55256

fda5c69 added code to avoid the less performant boxing of JSBoxedDartObject and instead use backend-specific logic to externalize and internalize the UniqueRef until we get a better solution from dart:js_interop. On the JS backends, this relied on casting to and from JSAny, as its representation type is just Object. However, this callback that takes in the UniqueRef as an arg is still typed as accepting JSBoxedDartObject, leading to a cast failure on the JS backends, since the representation type is JSObject.

The fix is to use JSAny. On dart2wasm, this makes no difference, as the underlying representation type does not change.

lib/web_ui/lib/src/engine/canvaskit/native_memory.dart

@@ -21,7 +21,7 @@ import 'package:ui/src/engine.dart';
 /// 5. The finalizer function is called with the SkPaint as the sole argument.
 /// 6. We call `delete` on SkPaint.
 DomFinalizationRegistry _finalizationRegistry = DomFinalizationRegistry(
-  (JSBoxedDartObject boxedUniq) {
+  (JSAny boxedUniq) {
     final UniqueRef<Object> uniq = boxedUniq.fromJSWrapper as UniqueRef<Object>;
     uniq.collect();
   }.toJS