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

pkg/compiler/lib/src/world.dart

Index: pkg/compiler/lib/src/world.dart
diff --git a/pkg/compiler/lib/src/world.dart b/pkg/compiler/lib/src/world.dart
index b8c6387727c151e8066117c8813110ef35291e8a..511af4cdb41fd5521fddd0daee9bea7bafb74eba 100644
--- a/pkg/compiler/lib/src/world.dart
+++ b/pkg/compiler/lib/src/world.dart
@@ -610,7 +610,7 @@ class World implements ClassWorld {
 
   bool fieldNeverChanges(Element element) {
     if (!element.isField) return false;
-    if (element.isNative) {
+    if (backend.isNative(element)) {
       // Some native fields are views of data that may be changed by operations.
       // E.g. node.firstChild depends on parentNode.removeBefore(n1, n2).
       // TODO(sra): Refine the effect classification so that native effects are