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

runtime/bin/builtin_sources.gni

Index: runtime/bin/builtin_sources.gni
diff --git a/runtime/bin/builtin_sources.gypi b/runtime/bin/builtin_sources.gni
similarity index 66%
rename from runtime/bin/builtin_sources.gypi
rename to runtime/bin/builtin_sources.gni
index fbf642e484976438dfb77d76797565bbe6c7083d..c9edd4060313561c68c62f4d20a0212b6c89a084 100644
--- a/runtime/bin/builtin_sources.gypi
+++ b/runtime/bin/builtin_sources.gni
@@ -1,10 +1,6 @@
-# Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+# Copyright (c) 2017, 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',
-  ],
-}
+builtin_sources = [ "builtin.dart" ]