Two related changes that both affect how cursor positions might change on single line edits.

dart/site/try/src/selection.dart

 // Copyright (c) 2014, 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.
 
 library trydart.selection;
 
 import 'dart:html' show
     CharacterData,
     Node,
     NodeFilter,
@@ skipping 41 matching lines @@
     if (anchorOffset >= 0) {
       selection.collapse(anchorNode, anchorOffset);
     }
   }
 
   void updateText(String newText) {
     text = newText;
     globalOffset = computeGlobalOffset(root, anchorNode, anchorOffset);
   }
 
+  TrySelection copyWithRoot(Node root) {
+    return new TrySelection.empty(root)
+        ..anchorNode = anchorNode
+        ..anchorOffset = anchorOffset;
+  }
+
   /// Computes the global offset, that is, the offset from [root].
   static int computeGlobalOffset(Node root, Node anchorNode, int anchorOffset) {
     if (anchorOffset == -1) return -1;
 
     int offset = 0;
     TreeWalker walker = new TreeWalker(root, NodeFilter.SHOW_TEXT);
     for (Node node = walker.nextNode();
          node != null;
          node = walker.nextNode()) {
       CharacterData text = node;
       if (anchorNode == text) {
         return anchorOffset + offset;
       }
       offset += text.data.length;
     }
 
     return -1;
   }
 }
 
 bool isCollapsed(Selection selection) {
   // Firefox and Chrome don't agree on if the selection is collapsed if there
   // is no node selected.
   return selection.isCollapsed && selection.anchorNode != null;
 }