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

lib/core/iterator.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.
 
 /**
  * The [Iterator] class provides methods to iterate over an object. It
  * is transparently used by the for-in construct to test for the end
  * of the iteration, and to get the elements.
  */
 interface Iterator<E> {
   /**
    * Gets the next element in the iteration. Throws a
    * [NoMoreElementsException] if no element is left.
    */
   E next();
 
   /**
    * Returns whether the [Iterator] has elements left.
    */
   bool hasNext();
 }