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

pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
index 5e817af18ab25d3fcd9d9254694c032e34e622eb..c26cf58a6252275701e3039169f68437cc5887e2 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -451,9 +451,9 @@ abstract class InferrerEngine<T, V extends TypeSystem>
   }
 
   bool isNativeElement(Element element) {
-    if (element.isNative) return true;
+    if (compiler.backend.isNative(element)) return true;
     return element.isClassMember
-        && element.enclosingClass.isNative
+        && compiler.backend.isNative(element.enclosingClass)
         && element.isField;
   }
 
@@ -555,7 +555,7 @@ class SimpleTypeInferrerVisitor<T>
       inferrer.setDefaultTypeOfParameter(element, type);
     });
 
-    if (analyzedElement.isNative) {
+    if (compiler.backend.isNative(analyzedElement)) {
       // Native methods do not have a body, and we currently just say
       // they return dynamic.
       return types.dynamicType;