Update context manager test after turning DOWN_CAST_COMPOSITE strong mode warning into a hint.

pkg/analysis_server/test/context_manager_test.dart

Index: pkg/analysis_server/test/context_manager_test.dart
diff --git a/pkg/analysis_server/test/context_manager_test.dart b/pkg/analysis_server/test/context_manager_test.dart
index ee33a76e9cef5cafcec42419d51d95e1931be38c..878d4392d9195467f5d89ea5a26adcc01650fb83 100644
--- a/pkg/analysis_server/test/context_manager_test.dart
+++ b/pkg/analysis_server/test/context_manager_test.dart
@@ -2331,10 +2331,9 @@ analyzer:
       AnalysisContext sdkContext = sourceFactory.dartSdk.context;
       expect(analysisOptions.strongMode, isTrue);
       expect(sdkContext.analysisOptions.strongMode, isTrue);
-      // The code is strong-mode clean, but we're using a Mock SDK that isn't
-      // configured correctly for strong mode so we get an error.
+      // The code is strong-mode clean.
       // Verify that TypeSystem was reset.
-      expect(context.computeErrors(testSource), hasLength(1));
+      expect(context.computeErrors(testSource), isEmpty);
     }
   }