[dart2js] Update lib/html/js_typed_interop_type_test to use external getters instead of final fields.
[dart2js] Update lib/html/js_typed_interop_type_test to use external
getters instead of final fields.

See discussion at #42955.

Change-Id: If8bb92f39eda2ee2cdb45e1eb07f1ef379ed0af3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157478
Auto-Submit: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>
Commit-Queue: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk