Relabel JSFunction as JSExportedDartFunction (#125453)

toDart exists on the latter, not the former.

Fixing https://github.com/flutter/flutter/pull/125220.

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packages/flutter/test/painting/_test_http_request.dart

@@ -76,7 +76,7 @@ class TestHttpRequest {
   JSVoid addEventListener(JSString type, DomEventListener listener) {
     if (type.toDart == mockEvent?.type) {
       final DartDomEventListener dartListener =
-        (listener as JSFunction).toDart as DartDomEventListener;
+        (listener as JSExportedDartFunction).toDart as DartDomEventListener;
       dartListener(mockEvent!.event);
     }
   }