move strong mode hints behind flag

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 e9b54974273f3d1e2e5d07956a5ea380ca7da9b9..330801f07f1f3f92c7a761a9ab9b4f7683810665 100644
--- a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
+++ b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
@@ -75,8 +75,9 @@ void testChecker(String name, Map<String, String> testFiles) {
         context.resolveCompilationUnit2(mainSource, mainSource);
 
     var collector = new _ErrorCollector();
-    var checker =
-        new CodeChecker(new TypeRules(context.typeProvider), collector);
+    var checker = new CodeChecker(
+        new TypeRules(context.typeProvider), collector,
+        hints: true);
 
     // Extract expectations from the comments in the test files, and
     // check that all errors we emit are included in the expected map.