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

lib/coreimpl/list.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A [List] is an indexable collection with a length. It can be of
  * fixed size or extendable.
  */
 class ListImplementation<E> {
   /**
@@ skipping 11 matching lines @@
    */
   factory List.from(Iterable<E> other) {
     // TODO(ajohnsen): Make external once the vm can handle it, so we don't
     // lose generic type information.
     // Issue: #4727
     return _from(other);
   }
 
   external static _from(Iterable<E> other);
 }