Move DualPivotQuicksort from coreimpl to core as _Sort.

tests/corelib/sort_test.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.
 
 // Dart test for sort routines.
 #library("SortTest.dart");
 #import("dart:coreimpl");
 #source("sort_helper.dart");
 
 main() {
   var compare = (a, b) => a.compareTo(b);
-  var sort = (list) => DualPivotQuicksort.sort(list, compare);
+  var sort = (list) => list.sort(compare);
   new SortHelper(sort, compare).run();
 
   compare = (a, b) => -a.compareTo(b);
   new SortHelper(sort, compare).run();
 
   compare = (a, b) => a.compareTo(b);
 
   // Pivot-canditate indices: 7, 15, 22, 29, 37
   // Test dutch flag partitioning (canditates 2 and 4 are the same).
   var list = [0, 0, 0, 0, 0, 0, 0, 0/**/, 0, 0, 0, 0, 0, 0, 0,
@@ skipping 16 matching lines @@
   // The second partition will be big (more than 2/3 of the list), and an
   // optimization kicks in that removes the pivots from the partition.
   list = [0, 9, 0, 9, 3, 9, 0, 1/**/, 1, 0, 1, 9, 8, 2, 1,
           1/**/, 4, 5, 2, 5, 0, 1, 8/**/, 8, 8, 5, 2, 2, 9, 8/**/,
           8, 4, 4, 1, 5, 3, 2, 8/**/, 5, 1, 2, 8, 5, 6, 8];
   list.sort(compare);
   Expect.listEquals(list, [0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2,
                            2, 2, 2, 2, 3, 3, 4, 4, 4, 5, 5, 5, 5, 5, 5,
                            6, 8, 8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9]);
 }