Reenable support for native factory constructors.

pkg/compiler/lib/src/native/enqueue.dart

Index: pkg/compiler/lib/src/native/enqueue.dart
diff --git a/pkg/compiler/lib/src/native/enqueue.dart b/pkg/compiler/lib/src/native/enqueue.dart
index 42ceba2d4b2f9f80c23f757dba1a6a137730675d..5fcea39d5353ce46847506be837b1ba411b6d159 100644
--- a/pkg/compiler/lib/src/native/enqueue.dart
+++ b/pkg/compiler/lib/src/native/enqueue.dart
@@ -348,7 +348,10 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
 
   registerElement(Element element) {
     reporter.withCurrentElement(element, () {
-      if (element.isFunction || element.isGetter || element.isSetter) {
+      if (element.isFunction ||
+          element.isFactoryConstructor ||
+          element.isGetter ||
+          element.isSetter) {
         handleMethodAnnotations(element);
         if (element.isNative) {
           registerMethodUsed(element);