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

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

Index: pkg/compiler/lib/src/js_backend/patch_resolver.dart
diff --git a/pkg/compiler/lib/src/js_backend/patch_resolver.dart b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
index c8e609de57a51583b4746a6cd03f57851578138a..60573ed2c9a36f116e7b26e60ad1c4c059236eca 100644
--- a/pkg/compiler/lib/src/js_backend/patch_resolver.dart
+++ b/pkg/compiler/lib/src/js_backend/patch_resolver.dart
@@ -31,7 +31,7 @@ class PatchResolverTask extends CompilerTask {
       });
       checkMatchingPatchSignatures(element, patch);
       element = patch;
-    } else if (!element.isJsInterop) {
+    } else if (!compiler.backend.isJsInterop(element)) {
       reporter.reportErrorMessage(
          element, MessageKind.PATCH_EXTERNAL_WITHOUT_IMPLEMENTATION);
     }