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

lib/coreimpl/dual_pivot_quicksort.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.
 
 /**
  * Dual-Pivot Quicksort algorithm.
  *
  * This class implements the dual-pivot quicksort algorithm as presented in
  * Vladimir Yaroslavskiy's paper.
  *
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     } else {
       // The second partition looks as follows:
       // [  *  |  >= pivot1 && <= pivot2  | * ]
       //        ^                        ^
       //       less                    great
       // Simply sort it by recursive descent.
       _doSort(a, less, great, compare);
     }
   }
 }