use proper notation for true, false, and null in doc comments

sdk/lib/collection/splay_tree.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.
 
 part of dart.collection;
 
 typedef bool _Predicate<T>(T value);
 
 /**
  * A node in a splay tree. It holds the sorting key and the left
@@ skipping 410 matching lines @@
 
   Iterable<K> get keys => new _SplayTreeKeyIterable<K>(this);
 
   Iterable<V> get values => new _SplayTreeValueIterable<K, V>(this);
 
   String toString() {
     return Maps.mapToString(this);
   }
 
   /**
-   * Get the first key in the map. Returns [null] if the map is empty.
+   * Get the first key in the map. Returns [:null:] if the map is empty.
    */
   K firstKey() {
     if (_root == null) return null;
     return _first.key;
   }
 
   /**
-   * Get the last key in the map. Returns [null] if the map is empty.
+   * Get the last key in the map. Returns [:null:] if the map is empty.
    */
   K lastKey() {
     if (_root == null) return null;
     return _last.key;
   }
 
   /**
    * Get the last key in the map that is strictly smaller than [key]. Returns
-   * [null] if no key was not found.
+   * [:null:] if no key was not found.
    */
   K lastKeyBefore(K key) {
     if (key == null) throw new ArgumentError(key);
     if (_root == null) return null;
     int comp = _splay(key);
     if (comp < 0) return _root.key;
     _SplayTreeNode<K> node = _root.left;
     if (node == null) return null;
     while (node.right != null) {
       node = node.right;
     }
     return node.key;
   }
 
   /**
    * Get the first key in the map that is strictly larger than [key]. Returns
-   * [null] if no key was not found.
+   * [:null:] if no key was not found.
    */
   K firstKeyAfter(K key) {
     if (key == null) throw new ArgumentError(key);
     if (_root == null) return null;
     int comp = _splay(key);
     if (comp > 0) return _root.key;
     _SplayTreeNode<K> node = _root.right;
     if (node == null) return null;
     while (node.left != null) {
       node = node.left;
@@ skipping 341 matching lines @@
 
   bool containsAll(Iterable<Object> other) {
     for (var element in other) {
       if (!this.contains(element)) return false;
     }
     return true;
   }
 
   void clear() { _clear(); }
 }