[dart2js] Migrate elements/indexed.dart to nnbd
[dart2js] Migrate elements/indexed.dart to nnbd

Change-Id: Ice865c35a5bc0641622da22c47cb5d56e782bcf3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/244600
Commit-Queue: Nate Biggs <4bdbe5a28604b503f0b7346ee4d58c334975887b@google.com>
Reviewed-by: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@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