Extract BackendUsage, MirrorsData, and CheckedModeHelpers from Backend.

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

Index: pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
index 6d4066e14875fd93b59a43ebffeee3d8902f3340..4afc814d6f1892fba033b81142fcdf84ea4b7c3b 100644
--- a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
@@ -99,7 +99,7 @@ class RuntimeTypeGenerator {
     /// dynamically at runtime. We also always generate tests against
     /// native classes.
     /// TODO(herhut): Generate tests for native classes dynamically, as well.
-    void generateIsTest(Element other) {
+    void generateIsTest(ClassElement other) {
       if (backend.isNative(classElement) || !classElement.isSubclassOf(other)) {
         result.properties[namer.operatorIs(other)] = js('1');
       }