Add getMinimum, getMaximum, getPreceding and getSucceeding to SplayTree.

tests/corelib/src/SplayTreeTest.dart

Index: tests/corelib/src/SplayTreeTest.dart
===================================================================
--- tests/corelib/src/SplayTreeTest.dart	(revision 4598)
+++ tests/corelib/src/SplayTreeTest.dart	(working copy)
@@ -24,6 +24,18 @@
       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));
   }
 }