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

corelib/src/iterable.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 [Iterable] interface allows to get an [Iterator] out of an
- * [Iterable] object.
- *
- * This interface is used by the for-in construct to iterate over an
- * [Iterable] object.
- * The for-in construct takes an [Iterable] object at the right-hand
- * side, and calls its [iterator] method to get an [Iterator] on it.
- *
- * A user-defined class that implements the [Iterable] interface can
- * be used as the right-hand side of a for-in construct.
- */
-interface Iterable<E> {
-  /**
-   * Returns an [Iterator] that iterates over this [Iterable] object.
-   */
-  Iterator<E> iterator();
-}