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

corelib/src/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();
-}