Introduce dart_sdk_lib_copy and make all sdk lib gypi files regular

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',
@@ skipping 14 matching lines @@
     'secure_server_socket.dart',
     'secure_socket.dart',
     'security_context.dart',
     'socket.dart',
     'stdio.dart',
     'string_transformer.dart',
     'websocket.dart',
     'websocket_impl.dart',
   ],
 }