Provide access to list of types that cannot be subtyped (issue 23125)

pkg/analyzer/lib/src/generated/testing/test_type_provider.dart

Index: pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
diff --git a/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart b/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
index da2b87b28432e6133881f4d80712e93defb87a6f..fe0c24bc7f942760574d1cb60247f2c39f1777ff 100644
--- a/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
+++ b/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
@@ -331,6 +331,16 @@ class TestTypeProvider implements TypeProvider {
   }
 
   @override
+  List<InterfaceType> get nonSubtypableTypes => <InterfaceType>[
+    nullType,
+    numType,
+    intType,
+    doubleType,
+    boolType,
+    stringType
+  ];
+
+  @override
   DartObjectImpl get nullObject {
     if (_nullObject == null) {
       _nullObject = new DartObjectImpl(nullType, NullState.NULL_STATE);