[tests, dart2js] Make test conditional on dart2js mode

Change-Id: Ibc1a99b4ad95e3df3b6fd85715cd8f0218235771
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332300
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

tests/language/this/as_covariant_call_checks_test.dart

@@ -44,6 +44,8 @@ void loop(A<String> obj, bool violateType) {
 }
 
 void main() {
+  if (dart2jsProductionMode) return; // All checks omitted in this mode.
+
   A<num>().field = 10;
   final obj = A<String>();
   loop(obj, false);