Move native and js interop properties from the element model to the JS backend

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

Index: pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
diff --git a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
index 707e7e7636929fe89ff84514e6d5a3e390f5bd20..70d597ef6bd21b18c982608b5eb1fc62019035f3 100644
--- a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
+++ b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
@@ -14,8 +14,8 @@ abstract class _MinifiedFieldNamer implements Namer {
   // this could be because the field belongs to a mixin. In such a case this
   // will return `null` and a normal field name has to be used.
   jsAst.Name _minifiedInstanceFieldPropertyName(Element element) {
-    if (element.hasFixedBackendName) {
-      return new StringBackedName(element.fixedBackendName);
+    if (backend.hasFixedBackendName(element)) {
+      return new StringBackedName(backend.getFixedBackendName(element));
     }
 
     _FieldNamingScope names;