Use Entity in native_data.

pkg/compiler/lib/src/js_backend/backend_usage.dart

Index: pkg/compiler/lib/src/js_backend/backend_usage.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_usage.dart b/pkg/compiler/lib/src/js_backend/backend_usage.dart
index cac4aa0e38be7e7dc6428b1da27b3e5d580b1968..1eb7887dc3d1bc1d081ed0d64783c2bf70a11f6e 100644
--- a/pkg/compiler/lib/src/js_backend/backend_usage.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_usage.dart
@@ -92,7 +92,7 @@ class BackendUsageBuilderImpl implements BackendUsageBuilder {
   final BackendHelpers _helpers;
   // TODO(johnniwinther): Remove the need for these.
   Setlet<FunctionEntity> _globalFunctionDependencies;
-  Setlet<ClassElement> _globalClassDependencies;
+  Setlet<ClassEntity> _globalClassDependencies;
 
   /// List of methods that the backend may use.
   final Set<FunctionEntity> _helperFunctionsUsed = new Set<FunctionEntity>();
@@ -248,7 +248,7 @@ class BackendUsageBuilderImpl implements BackendUsageBuilder {
   void registerGlobalClassDependency(ClassEntity element) {
     assert(element != null);
     if (_globalClassDependencies == null) {
-      _globalClassDependencies = new Setlet<ClassElement>();
+      _globalClassDependencies = new Setlet<ClassEntity>();
     }
     _globalClassDependencies.add(element);
   }