Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/js_emitter/type_test_registry.dart

Index: pkg/compiler/lib/src/js_emitter/type_test_registry.dart
diff --git a/pkg/compiler/lib/src/js_emitter/type_test_registry.dart b/pkg/compiler/lib/src/js_emitter/type_test_registry.dart
index 15f3b5562f3f124f8ba8dd3bc5c853c459b4aac9..c8e6b19d1df052783114440780f7f872f58d81aa 100644
--- a/pkg/compiler/lib/src/js_emitter/type_test_registry.dart
+++ b/pkg/compiler/lib/src/js_emitter/type_test_registry.dart
@@ -136,7 +136,8 @@ class TypeTestRegistry {
     }
 
     bool canBeReified(Element element) {
-      return (canTearOff(element) || backend.isAccessibleByReflection(element));
+      return (canTearOff(element) ||
+          backend.mirrorsData.isAccessibleByReflection(element));
     }
 
     // Find all types referenced from the types of elements that can be