Use ShadowRoot to hide non-user content (for example, diagnostics).

dart/site/try/src/html_to_text.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.htmlToText;
 
 import 'dart:math' show
     max;
 
 import 'dart:html' show
     Element,
     Node,
     NodeFilter,
+    ShadowRoot,
     Text,
     TreeWalker;
 
 import 'selection.dart' show
     TrySelection;
 
 /// Returns true if [node] is a block element, that is, not inline.
 bool isBlockElement(Node node) {
   if (node is! Element) return false;
   Element element = node;
@@ skipping 30 matching lines @@
       case Node.CDATA_SECTION_NODE:
       case Node.TEXT_NODE:
         if (selection.anchorNode == node) {
           selectionOffset = selection.anchorOffset + buffer.length;
         }
         Text text = node;
         buffer.write(text.data.replaceAll('\xA0', ' '));
         break;
 
       default:
-        if (node.nodeName == 'BR') {
+        if (!ShadowRoot.supported &&
+            node is Element &&
+            node.getAttribute('try-dart-shadow-root') != null) {
+          skip(node, walker);
+        } else if (node.nodeName == 'BR') {
           buffer.write('\n');
         } else if (node != root && isBlockElement(node)) {
           selectionOffset =
               max(selectionOffset, htmlToText(node, buffer, selection));
           skip(node, walker);
         }
         break;
     }
   }
 
   if (isBlockElement(root)) {
     buffer.write('\n');
   }
 
   return selectionOffset;
 }