Move SplayTreeMap from coreimpl to collection.

tests/corelib/splay_tree_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 Splaytrees.
 #library("SplayTreeTest.dart");
-#import("dart:coreimpl");
+#import("dart:collection");
 
 
 class SplayTreeMapTest {
 
   static testMain() {
     SplayTreeMap tree = new SplayTreeMap();
     tree[1] = "first";
     tree[3] = "third";
     tree[5] = "fifth";
     tree[2] = "second";
@@ skipping 30 matching lines @@
     Expect.equals(null, tree.firstKeyAfter(7));
 
     Expect.equals(5, tree.lastKeyBefore(6));
     Expect.equals(7, tree.firstKeyAfter(6));
   }
 }
 
 main() {
   SplayTreeMapTest.testMain();
 }