linter: collection_methods_unrelated_type: Null not related to non-nullable

Fixes https://github.com/dart-lang/sdk/issues/57101

This change affects collection_methods_unrelated_type and
unrelated_type_equality_checks. For example these are now reported:

```dart
void f(Set<String> p1, int p2) {
  p1.contains(null);
  p2 == null;
}
```

Change-Id: Ib58bad2beb6b9df8fe7c71f27213055e9b8d5cb9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/489481
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/linter/lib/src/util/dart_type_utilities.dart

@@ -182,6 +182,12 @@ bool typesAreUnrelated(
   }
   var promotedLeftType = typeSystem.promoteToNonNull(leftType);
   var promotedRightType = typeSystem.promoteToNonNull(rightType);
+  if (leftType.isDartCoreNull && typeSystem.isNonNullable(rightType)) {
+    return true;
+  }
+  if (rightType.isDartCoreNull && typeSystem.isNonNullable(leftType)) {
+    return true;
+  }
   if (promotedLeftType == promotedRightType ||
       typeSystem.isSubtypeOf(promotedLeftType, promotedRightType) ||
       typeSystem.isSubtypeOf(promotedRightType, promotedLeftType)) {

pkg/linter/test/rules/collection_methods_unrelated_type_test.dart

@@ -65,7 +65,23 @@ var x = <M>[].contains(C());
   }
 
   test_contains_related_null() async {
-    await assertNoDiagnostics('var x = <num>[].contains(null);');
+    await assertNoDiagnostics('var x = <num?>[].contains(null);');
+  }
+
+  test_contains_related_null_genericNullable() async {
+    await assertNoDiagnostics('''
+bool f<T extends Object?>() {
+  return <T>[].contains(null);
+}
+''');
+  }
+
+  test_contains_related_null_genericNullable2() async {
+    await assertNoDiagnostics('''
+bool f<T extends Object>() {
+  return <T?>[].contains(null);
+}
+''');
   }
 
   test_contains_related_Object() async {
@@ -164,6 +180,21 @@ abstract class C implements List<num> {
     );
   }
 
+  test_contains_unrelated_null() async {
+    await assertDiagnostics('var x = <num>[].contains(null);', [lint(25, 4)]);
+  }
+
+  test_contains_unrelated_null_generic() async {
+    await assertDiagnostics(
+      '''
+bool f<T extends Object>() {
+  return <T>[].contains(null);
+}
+''',
+      [lint(53, 4)],
+    );
+  }
+
   test_contains_unrelated_recordAndNonRecord() async {
     await assertDiagnostics("var x = <(int, int)>[].contains('hi');", [
       lint(32, 4),