[dart2js] pass null for missing required argument in weak mode

Now passing:
language/nnbd/required_named_parameters/required_named_args_legacy_test
language/nnbd/required_named_parameters/required_named_args_weak_test

Change-Id: Ide909eb6eb996ca48c851b45f1069f6562b52d5e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151096
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

Author: rakudrama

pkg/compiler/lib/src/js_model/env.dart

@@ -648,13 +648,17 @@ abstract class FunctionDataForEachParameterMixin implements FunctionData {
       String name = parameter.name;
       ConstantValue defaultValue;
       if (parameter.isRequired) {
-        defaultValue = elementMap.getRequiredSentinelConstantValue();
+        if (elementMap.types.useLegacySubtyping) {
+          defaultValue = NullConstantValue();
+        } else {
+          defaultValue = elementMap.getRequiredSentinelConstantValue();
+        }
       } else if (isOptional) {
         if (parameter.initializer != null) {
           defaultValue =
               elementMap.getConstantValue(memberContext, parameter.initializer);
         } else {
-          defaultValue = new NullConstantValue();
+          defaultValue = NullConstantValue();
         }
       }
       f(type, name, defaultValue);