Move core and coreimpl from corelib/ to lib/core and lib/coreimpl.

lib/coreimpl/options.dart

 // 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.
 
 class RuntimeOptions implements Options {
   List<String> get arguments() {
     if (_arguments === null) {
       // On first access make a copy of the native arguments.
       _arguments = _nativeArguments.getRange(0, _nativeArguments.length);
     }
@@ skipping 12 matching lines @@
 
   // This arguments singleton is written to by the embedder if applicable.
   static List<String> _nativeArguments = const [];
 
   // This executable singleton is written to by the embedder if applicable.
   static String _nativeExecutable = '';
 
   // This script singleton is written to by the embedder if applicable.
   static String _nativeScript = '';
 }