[dart2js] Never elide _Enum.index

This ensures that `_Enum.index` is always available for switch strength
reduction optimizations. Most real programs use the index of at least one `enum`, so the field is not usually elided. This change makes small tests and benchmarks behave more like real programs where these optimizations happen.

Bug: #51657
CoreLibraryReviewExempt: dart2js specific annotation
Change-Id: If98e483d7cce265e823fd574565089e328215cca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446481
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>

benchmarks/SwitchFSM/dart/SwitchFSM.dart

@@ -136,12 +136,6 @@ class Benchmark extends BenchmarkBase {
 enum SomeEnum { element }
 
 void main() {
-  // TODO(http://dartbug.com/51657): dart2js will remove `_Enum.index` in simple
-  // programs that don't appear to use the field. This defeats the enum-switch
-  // optimization that works more reliably in larger programs. Remove this code
-  // that marks `_Enum.index` as used when #51657 is fixed.
-  Expect.equals(0, SomeEnum.element.index);
-
   final benchmarks = [
     Benchmark('enum', match_enum.match),
     Benchmark('int', match_int.match),