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..3a034980a2c8949d0324ff328f77a34f24928de9 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -446,7 +446,13 @@ 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 not "loaded", but doesn't have a deferred prefix.

[siva, 2017/03/17 21:07:05]

The comment here seems opposite to the condition importee.Loaded()?

[rmacnak, 2017/03/17 22:03:31]

The first half of the condition gets eagerly loaded libraries or deferred
libraries that have had loadLibrary called. The second half picks up native
extensions.  Clarified in the comment.

+    // 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()));