[infra] Translate _sources.gypi files to _sources.gni files

runtime/bin/builtin_sources.gypi

Index: runtime/bin/builtin_sources.gypi
diff --git a/runtime/bin/builtin_sources.gypi b/runtime/bin/builtin_sources.gypi
deleted file mode 100644
index fbf642e484976438dfb77d76797565bbe6c7083d..0000000000000000000000000000000000000000
--- a/runtime/bin/builtin_sources.gypi
+++ /dev/null
@@ -1,10 +0,0 @@
-# Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
-# for details. All rights reserved. Use of this source code is governed by a
-# BSD-style license that can be found in the LICENSE file.
-
-# This file contains all dart sources for the dart:_builtin library.
-{
-  'sources': [
-    'builtin.dart',
-  ],
-}