Mark native functions when they are created.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
===================================================================
--- runtime/vm/class_finalizer.cc	(revision 30796)
+++ runtime/vm/class_finalizer.cc	(working copy)
@@ -1188,6 +1188,7 @@
                             /* is_const = */ field.is_const(),
                             /* is_abstract = */ false,
                             /* is_external = */ false,
+                            /* is_native = */ false,
                             cls,
                             field.token_pos()));
           getter.set_result_type(type);
@@ -1798,6 +1799,7 @@
                         false,  // Not const.
                         false,  // Not abstract.
                         false,  // Not external.
+                        false,  // Not native.
                         mixin_app,
                         mixin_app.token_pos()));