This adds the flag `isEnumElement` to the `Field` node and uses it
to recognize enum elements for the exhaustiveness computation on
switch statements.
Closes#49697
TEST=pkg/front_end/testcases/general/issue49697/main.dart
Change-Id: I21852f3b063329e043639c825054f538c8ed6536
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/255808
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Johnni Winther
dcaebaa793