fix prefer_void_to_null false positive on extension type representa…

…tions

Fixes: https://github.com/dart-lang/linter/issues/4720

Change-Id: Ie3a736cbd4d190135eb77932ec8ba9352683226e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/330200
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

pkg/linter/lib/src/rules/prefer_void_to_null.dart

@@ -142,6 +142,11 @@ class _Visitor extends SimpleAstVisitor<void> {
       return;
     }
 
+    // extension type N(Null _) ...
+    if (parent is RepresentationDeclaration) {
+      return;
+    }
+
     // https://github.com/dart-lang/linter/issues/2792
     if (parent is MethodDeclaration &&
         isVoidIncompatibleOverride(parent, node)) {

pkg/linter/test/rules/prefer_void_to_null_test.dart

@@ -39,6 +39,14 @@ void f(int a) {
     ]);
   }
 
+  /// https://github.com/dart-lang/linter/issues/4759
+  test_extensionTypeRepresentation() async {
+    await assertNoDiagnostics(r'''
+extension type B<T>(T? _) {}
+extension type N(Null _) implements B<Never> {}
+''');
+  }
+
   test_localVariable() async {
     await assertNoDiagnostics(r'''
 void f() {