Don't loop forever on scanner errors.

dart/site/try/src/interaction_manager.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.interaction_manager;
 
 import 'dart:html';
 
 import 'dart:convert' show
     JSON;
@@ skipping 242 matching lines @@
         Decoration decoration = editor.getDecoration(token);
         if (decoration == null) continue;
         bool hasSelection = false;
         int selectionOffset = selection.anchorOffset;
 
         if (selection.isCollapsed && selection.anchorNode == node) {
           hasSelection = true;
           selectionOffset = selection.anchorOffset;
         }
         int splitPoint = token.charOffset - offset;
+        if (splitPoint < 0) continue; // Happens for scanner errors.
         Text str = node.splitText(splitPoint);
         Text after = str.splitText(token.charCount);
         offset += splitPoint + token.charCount;
         mainEditorPane.insertBefore(after, node.nextNode);
         mainEditorPane.insertBefore(decoration.applyTo(str), after);
 
         if (hasSelection && selectionOffset > node.length) {
           selectionOffset -= node.length;
           if (selectionOffset > str.length) {
             selectionOffset -= str.length;
@@ skipping 319 matching lines @@
       event.getModifierState("Fn") ||
       event.getModifierState("Meta") ||
       event.getModifierState("NumLock") ||
       event.getModifierState("ScrollLock") ||
       event.getModifierState("Scroll") ||
       event.getModifierState("Win") ||
       event.getModifierState("Shift") ||
       event.getModifierState("SymbolLock") ||
       event.getModifierState("OS");
 }