[CQ] linter: enable unnecessary_underscores

Change-Id: I34532e491f941c9287d5e92ccaa96795251067a1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404344
Auto-Submit: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: pq

pkg/linter/analysis_options.yaml

@@ -57,6 +57,7 @@ linter:
     - unnecessary_null_checks
     - unnecessary_parenthesis
     - unnecessary_statements
+    - unnecessary_underscores
     - unreachable_from_main
     - use_if_null_to_convert_nulls_to_bools
     - use_late_for_private_fields_and_variables