[dart2js] remove unused helpers
[dart2js] remove unused helpers

These helper methods were used by generated code long ago, before
Dart 2 I believe.  There appears to be no uses of these methods in
our code generation today (last use in the compiler was removed in https://dart-review.googlesource.com/c/sdk/+/75300/2/pkg/compiler/lib/src/common_elements.dart 4 years ago), so I believe we can remove them.

Change-Id: I09647000e4b2643a54246cb77a6d0d06b78abeb3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/260961
Commit-Queue: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Reviewed-by: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@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