don't overreport prefer_void_to_null on cast patterns and as expressions

lib/src/rules/prefer_void_to_null.dart

@@ -125,9 +125,13 @@ class _Visitor extends SimpleAstVisitor<void> {
     var parent = node.parent;
 
     // Null Function()
-    if (parent is GenericFunctionType) {
-      return;
-    }
+    if (parent is GenericFunctionType) return;
+
+    // case var _ as Null
+    if (parent is CastPattern) return;
+
+    // a as Null
+    if (parent is AsExpression) return;
 
     // Function(Null)
     if (parent is SimpleFormalParameter &&

test/rules/prefer_void_to_null_test.dart

@@ -18,21 +18,23 @@ class PreferVoidToNullTestLanguage300 extends LintRuleTest
   @override
   String get lintRule => 'prefer_void_to_null';
 
+  /// https://github.com/dart-lang/linter/issues/4201
+  test_castAsExpression() async {
+    await assertNoDiagnostics(r'''
+void f(int a) {
+  a as Null;
+}
+''');
+  }
+
   /// https://github.com/dart-lang/linter/issues/4201
   test_castPattern() async {
-    await assertDiagnostics(r'''
+    await assertNoDiagnostics(r'''
 void f(int a) {
   switch (a) {
-    case var _ as void:
+    case var _ as Null:
   }
-  a as void;
 }
-''', [
-      // No lint.
-      error(WarningCode.UNNECESSARY_CAST_PATTERN, 46, 2),
-      error(ParserErrorCode.EXPECTED_TYPE_NAME, 49, 4),
-      error(HintCode.UNNECESSARY_CAST, 61, 9),
-      error(ParserErrorCode.EXPECTED_TYPE_NAME, 66, 4),
-    ]);
+''');
   }
 }