Avoid issuing incorrect errors when super mixins are enabled

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

Index: pkg/analyzer/test/src/task/strong/strong_test_helper.dart
diff --git a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
index c4c461e97240fc5edf097ea18ac4f2e7b98396ab..a330f567857918b7135138fca5343dfbe782db16 100644
--- a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
+++ b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
@@ -273,8 +273,8 @@ class AbstractStrongTest {
       {bool declarationCasts: true,
       bool implicitCasts: true,
       bool implicitDynamic: true,
-      List<String> nonnullableTypes:
-          AnalysisOptionsImpl.NONNULLABLE_TYPES}) async {
+      List<String> nonnullableTypes: AnalysisOptionsImpl.NONNULLABLE_TYPES,
+      bool superMixins: false}) async {
     _checkCalled = true;
 
     File mainFile =
@@ -288,6 +288,7 @@ class AbstractStrongTest {
     analysisOptions.implicitCasts = implicitCasts;
     analysisOptions.implicitDynamic = implicitDynamic;
     analysisOptions.nonnullableTypes = nonnullableTypes;
+    analysisOptions.enableSuperMixins = superMixins;
 
     var mockSdk = new MockSdk(resourceProvider: _resourceProvider);
     mockSdk.context.analysisOptions = analysisOptions;