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

sdk/lib/mirrors/mirrors_sources.gypi

Index: sdk/lib/mirrors/mirrors_sources.gypi
diff --git a/sdk/lib/mirrors/mirrors_sources.gypi b/sdk/lib/mirrors/mirrors_sources.gypi
deleted file mode 100644
index 8c97af59cd5a40fd8a913fe8746b9880661d189c..0000000000000000000000000000000000000000
--- a/sdk/lib/mirrors/mirrors_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.
-
-{
-  'sources': [
-    'mirrors.dart',
-    # The above file needs to be first if additional parts are added to the lib.
-  ],
-}