xilem_web: Fix issue with downcasting to a wrong type transitively from `AnyView` #1098

Job	Run time
cargo check (msrv) (wasm32)	16s
cargo check (msrv) (windows-latest)	1m 41s
cargo clippy (windows-latest)	2m 55s
cargo check (aarch64-android)	18s
cargo check (msrv) (macos-latest)	29s
cargo clippy (macos-latest)	59s
cargo check (msrv) (ubuntu-latest)	32s
cargo clippy (wasm32)	54s
cargo clippy (ubuntu-latest)	1m 9s
cargo doc	1m 26s
formatting	43s
Prime LFS Cache	4s
cargo test (wasm32)	21s
typos	5s
cargo test (windows-latest)	2m 41s
cargo test (macos-latest)	1m 3s
cargo test (ubuntu-latest)	2m 48s
	18m 24s

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