Add non-null types list to StrongModeOptions

pkg/analyzer/test/src/task/strong/non_null_checker_test.dart

Index: pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart b/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
index 1b2a7a6de77f6e864c07d9e3ce6365c4a12c8290..7b9b933973941f16ec1a328330c89e10378b925a 100644
--- a/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
+++ b/pkg/analyzer/test/src/task/strong/non_null_checker_test.dart
@@ -42,4 +42,16 @@ main() {
 }
 ''');
   }
+
+  void test_nullableTypes() {
+    // By default x can be set to null.
+    checkFile('''int x = null;''');
+  }
+
+  @failingTest
+  void test_nonnullableTypes() {
+    // If `int`s are non-nullable, then this code should throw an error.
+    addFile('''int x = /*error:INVALID_ASSIGNMENT*/null;''');
+    check(nonnullableTypes: <String>['dart:core,int']);
+  }
 }