Fix bug in SplayTreeSet when removeWhere removes all elements.

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 505 matching lines @@
   _SplayTreeIterator(_SplayTree tree)
       : _tree = tree,
         _modificationCount = tree._modificationCount,
         _splayCount = tree._splayCount {
     _findLeftMostDescendent(tree._root);
   }
 
   _SplayTreeIterator.startAt(_SplayTree tree, var startKey)
       : _tree = tree,
         _modificationCount = tree._modificationCount {
+    if (tree._root == null) return;
     int compare = tree._splay(startKey);
     _splayCount = tree._splayCount;
     if (compare < 0) {
       // Don't include the root, start at the next element after the root.
       _findLeftMostDescendent(tree._root.right);
     } else {
       _workList.add(tree._root);
     }
   }
 
@@ skipping 282 matching lines @@
 
   bool containsAll(Iterable<Object> other) {
     for (var element in other) {
       if (!this.contains(element)) return false;
     }
     return true;
   }
 
   void clear() { _clear(); }
 }