Introduce dart_sdk_lib_copy and make all sdk lib gypi files regular

sdk/lib/developer/developer_sources.gypi

 # Copyright (c) 2015, 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': [
     'developer.dart',
+    # The above file needs to be first if additional parts are added to the lib.
     'extension.dart',
     'profiler.dart',
     'timeline.dart',
-    # The above file needs to be first if additional parts are added to the lib.
   ],
 }