Report MISSING_DEFAULT_VALUE_FOR_PARAMETER only when NNBD is enabled.

It works now, because we consider types in opt-out libraries as S*,
and so far TypsSystem.isPotentiallyNonNullable(S*) returns false,
because TypeSystem.isNullable(S*) returns true. But I think this
is wrong. The spec says:

We say that a type T is nullable if Null <: T. This is equivalent to
the syntactic criterion that T is any of:
<cut>
- S* for some S where S is nullable
<cut>

  So, we currently say that `int*` is nullable.
  But this is wrong - `int` is not nullable.

R=brianwilkerson@google.com, paulberry@google.com

Change-Id: I87340fbed23cebd3d68c92579b7d1785b2fc097a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106952
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -6059,6 +6059,8 @@ class ErrorVerifier extends RecursiveAstVisitor<void> {
   }
 
   void _checkUseOfDefaultValuesInParameters(FormalParameterList node) {
+    if (!_isNonNullable) return;
+
     AstNode parent = node.parent;
     if (parent is FieldFormalParameter ||
         parent is FunctionTypeAlias ||