Use strict equality when comparing with null, especially when null is a default value.

corelib/src/implementation/splay_tree.dart

Index: corelib/src/implementation/splay_tree.dart
===================================================================
--- corelib/src/implementation/splay_tree.dart	(revision 855)
+++ corelib/src/implementation/splay_tree.dart	(working copy)
@@ -187,7 +187,7 @@
         current = current.left;
       } else {
         f(current.key, current.value);
-        while (current.right == null) {
+        while (current.right === null) {
           if (list.isEmpty()) return;
           current = list.removeLast();
           f(current.key, current.value);