[dart2js] ignore --use-old-rti option
[dart2js] ignore --use-old-rti option

The flag won't work properly after unforking the NNBD work.

Once the unfork stablizes we can remove lots of code.

Change-Id: I52a1c49208ac2dd78a38af79cf5f803de5d02ed1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/147000
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Commit-Queue: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@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