Move SplayTreeMap from coreimpl to collection.

tests/html/postmessage_structured_test.dart

 // Copyright (c) 2012, 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
 
 #library('postmessage_js_test');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
-#import('dart:coreimpl');  // SplayTreeMap
+#import('dart:collection');  // SplayTreeMap
 #import('utils.dart');
 
 injectSource(code) {
   final script = new ScriptElement();
   script.type = 'text/javascript';
   script.innerHTML = code;
   document.body.nodes.add(script);
 }
 
 main() {
@@ skipping 111 matching lines @@
   go('test_map', obj1);
   go('test_DAG', obj2);
   go('test_cycle', obj3);
   go('test_simple_splay', obj4);
   go('const_array_1', const [const [1], const [2]]);
   go('const_array_dag', const [const [1], const [1]]);
   go('array_deferred_copy', [1,2,3, obj3, obj3, 6]);
   go('array_deferred_copy_2', [1,2,3, [4, 5, obj3], [obj3, 6]]);
   go('cyclic_list', cyclic_list);
 }