Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index 7c5ab6a3a7c22f24eb762a19c7b79f55621e0463..8b9c125dbcd34b93d782e3461b60aa55ca18bb10 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -1072,7 +1072,7 @@ class ClosureTranslator extends Visitor {
     closedWorldRefiner.registerClosureClass(globalizedElement);
     FunctionElement callElement = new SynthesizedCallMethodElementX(
         Identifiers.call, element, globalizedElement, node, elements);
-    backend.maybeMarkClosureAsNeededForReflection(
+    backend.mirrorsData.maybeMarkClosureAsNeededForReflection(
         globalizedElement, callElement, element);
     MemberElement enclosing = element.memberContext;
     enclosing.nestedClosures.add(callElement);