Fix LibraryDependencyMirrors when the target is a native extension.

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index 4a68c8667877c15dfb49755e136314e04e47ec3f..1dade418b20d37dcac6e91577b4a9dfe5a4c9293 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -446,7 +446,14 @@ static RawInstance* CreateLibraryDependencyMirror(Thread* thread,
 
   const Array& args = Array::Handle(Array::New(7));
   args.SetAt(0, importer);
-  args.SetAt(1, importee.Loaded() ? importee_mirror : prefix);
+  if (importee.Loaded() || prefix.IsNull()) {
+    // A native extension is never "loaded" by the embedder. Use the fact that
+    // it doesn't have an prefix where asa  deferred import does to distinguish
+    // it from a deferred import. It will appear like an empty library.
+    args.SetAt(1, importee_mirror);
+  } else {
+    args.SetAt(1, prefix);
+  }
   args.SetAt(2, combinators);
   args.SetAt(3, prefix.IsNull() ? Object::null_object()
                                 : String::Handle(prefix.name()));