Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

tests/compiler/dart2js/mirrors_used_test.dart

Index: tests/compiler/dart2js/mirrors_used_test.dart
diff --git a/tests/compiler/dart2js/mirrors_used_test.dart b/tests/compiler/dart2js/mirrors_used_test.dart
index 39755167542fc32d78a337b23370429821a7e6ab..150bfb37904df42dfb93c16d3bd63e1dba61810a 100644
--- a/tests/compiler/dart2js/mirrors_used_test.dart
+++ b/tests/compiler/dart2js/mirrors_used_test.dart
@@ -109,15 +109,18 @@ void main() {
       library.forEachLocalMember((member) {
         if (library == compiler.mainApp && member.name == 'Foo') {
           Expect.isTrue(
-              compiler.backend.isAccessibleByReflection(member), '$member');
+              compiler.backend.mirrorsData.isAccessibleByReflection(member),
+              '$member');
           member.forEachLocalMember((classMember) {
             Expect.isTrue(
-                compiler.backend.isAccessibleByReflection(classMember),
+                compiler.backend.mirrorsData
+                    .isAccessibleByReflection(classMember),
                 '$classMember');
           });
         } else {
           Expect.isFalse(
-              compiler.backend.isAccessibleByReflection(member), '$member');
+              compiler.backend.mirrorsData.isAccessibleByReflection(member),
+              '$member');
         }
       });
     }