[dart2js] improve nullsafe API for a few _find* methods
[dart2js] improve nullsafe API for a few _find* methods

A few private methods in common/elements.dart are used to lookup classes
and members. This is a step in normalizing these methods, so that we use
different APIs when the values are required and when they are not.

Ideally this is a change we'd do more generally in our APIs (e.g. the env APIs
should be split).


Change-Id: Iacc2f012f6f60ae960c61d8897630f6a7f1d60e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260080
Commit-Queue: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Reviewed-by: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@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