[dart:html] Migrate htmlcollection_test to null-safe
[dart:html] Migrate htmlcollection_test to null-safe

Change-Id: I68a7874cc0afda861cb002901f7db0b58ad4be38
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142266
Reviewed-by: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@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