[dart2js] migrate kernel/element_map.dart
[dart2js] migrate kernel/element_map.dart

Change-Id: I64d3cfa518ec4226c8008c3e9cab3d2f631e8934
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260286
Reviewed-by: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>
Reviewed-by: Nate Biggs <4bdbe5a28604b503f0b7346ee4d58c334975887b@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