add network_policy to io gni

This source file is missing from the gni that flutter engine uses to
construct the analyzer summary. Assuming it wasn't left out intentionally.

Bug: https://github.com/flutter/flutter/issues/76228
Change-Id: I9758ed5839d5b3898064d0372798ecfaa70f9042
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/185489
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Zach Anderson <zra@google.com>
diff --git a/sdk/lib/io/io_sources.gni b/sdk/lib/io/io_sources.gni
index 7410a13..fb148df 100644
--- a/sdk/lib/io/io_sources.gni
+++ b/sdk/lib/io/io_sources.gni
@@ -20,6 +20,7 @@
   "io_sink.dart",
   "link.dart",
   "namespace_impl.dart",
+  "network_policy.dart",
   "network_profiling.dart",
   "overrides.dart",
   "platform.dart",