Add getMinimum, getMaximum, getPreceding and getSucceeding to SplayTree.

tests/corelib/src/SplayTreeTest.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");
 
 
 class SplayTreeTest {
 
   static testMain() {
     SplayTree tree = new SplayTree();
     tree[1] = "first";
     tree[3] = "third";
     tree[5] = "fifth";
     tree[2] = "second";
     tree[4] = "fourth";
 
     var correctSolution = ["first", "second", "third", "fourth", "fifth"];
 
     tree.forEach((key, value) {
       Expect.equals(true, key >= 1);
       Expect.equals(true, key <= 5);
       Expect.equals(value, correctSolution[key - 1]);
     });
+
+    Expect.equals(1, tree.firstKey());
+    Expect.equals(1, tree.firstKey());
+
+    Expect.equals(2, tree.keyBefore(3));
+    Expect.equals(4, tree.keyAfter(3));
+
+    Expect.equals(-10, tree.keyBefore(1, -10));
+    Expect.equals(2, tree.keyAfter(1, -10));
+
+    Expect.equals(4, tree.keyBefore(5, -10));
+    Expect.equals(-10, tree.keyAfter(5, -10));
   }
 }
 
 main() {
   SplayTreeTest.testMain();
 }