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

sdk/lib/io/io_sources.gypi

-# 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': [
-    'io.dart',
-    # The above file needs to be first if additional parts are added to the lib.
-    'bytes_builder.dart',
-    'common.dart',
-    'crypto.dart',
-    'data_transformer.dart',
-    'directory.dart',
-    'directory_impl.dart',
-    'eventhandler.dart',
-    'file.dart',
-    'file_impl.dart',
-    'file_system_entity.dart',
-    'http.dart',
-    'http_date.dart',
-    'http_headers.dart',
-    'http_impl.dart',
-    'http_parser.dart',
-    'http_session.dart',
-    'io_resource_info.dart',
-    'io_sink.dart',
-    'io_service.dart',
-    'link.dart',
-    'platform.dart',
-    'platform_impl.dart',
-    'process.dart',
-    'service_object.dart',
-    'secure_server_socket.dart',
-    'secure_socket.dart',
-    'security_context.dart',
-    'socket.dart',
-    'stdio.dart',
-    'string_transformer.dart',
-    'sync_socket.dart',
-    'websocket.dart',
-    'websocket_impl.dart',
-  ],
-}