Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index 4b5c2ccad1b551a29495f93f4e95e82475e91e45..57db6e9c9e56f6d90dbcfc0b7b2433674230bc42 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -565,7 +565,7 @@ class DeferredLoadTask extends CompilerTask {
       // asked isNeededForReflection. Instead an internal error is triggered.
       // So we have to filter them out here.
       if (element is AnalyzableElementX && !element.hasTreeElements) return;
-      if (compiler.backend.isAccessibleByReflection(element)) {
+      if (compiler.backend.mirrorsData.isAccessibleByReflection(element)) {
         _mapDependencies(
             element: element, import: deferredImport, isMirrorUsage: true);
       }