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

pkg/compiler/lib/src/ssa/interceptor_simplifier.dart

Index: pkg/compiler/lib/src/ssa/interceptor_simplifier.dart
diff --git a/pkg/compiler/lib/src/ssa/interceptor_simplifier.dart b/pkg/compiler/lib/src/ssa/interceptor_simplifier.dart
index 97b4b176a703f4eaa7e29c9599231481db65287a..d61c49dc5189f817f6eace8d9924be06c79728d1 100644
--- a/pkg/compiler/lib/src/ssa/interceptor_simplifier.dart
+++ b/pkg/compiler/lib/src/ssa/interceptor_simplifier.dart
@@ -167,7 +167,7 @@ class SsaSimplifyInterceptors extends HBaseVisitor
       // code is completely insensitive to the specific instance subclasses, we
       // can use the non-leaf class directly.
       ClassElement element = type.singleClass(classWorld);
-      if (element != null && element.isNative) {
+      if (element != null && backend.isNative(element)) {
         return element;
       }
     }