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

dart/site/try/src/decoration.dart

 // Copyright (c) 2013, 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.decoration;
 
 import 'dart:html';
 
+import 'shadow_root.dart' show
+    setShadowRoot;
+
 class Decoration {
   final String color;
   final bool bold;
   final bool italic;
   final bool stress;
   final bool important;
 
   const Decoration({this.color: '#000000',
                     this.bold: false,
                     this.italic: false,
@@ skipping 44 matching lines @@
       tip = error(message);
     }
     return element..append(
         new AnchorElement()
             ..classes.add('diagnostic')
             ..nodes.addAll(nodes)
             ..append(tip));
   }
 }
 
-info(text) {
-  if (text is String) {
-    text = new Text(text);
+createAlert(text, [String cls]) {
+  var classes = ['alert'];
+  if (cls != null) {
+    classes.add(cls);
   }
-  return new SpanElement()
-      ..classes.addAll(['alert', 'alert-info'])
-      ..style.opacity = '0.75'
-      ..append(text);
+  SpanElement result = new SpanElement()
+      ..classes.addAll(classes)
+      ..style.opacity = '0.75';
+  setShadowRoot(result, text);
+  return result;
 }
 
-error(text) {
-  if (text is String) {
-    text = new Text(text);
-  }
-  return new SpanElement()
-      ..classes.addAll(['alert', 'alert-error'])
-      ..style.opacity = '0.75'
-      ..append(text);
-}
+info(text) => createAlert(text, 'alert-info');
 
-warning(text) {
-  if (text is String) {
-    text = new Text(text);
-  }
-  return new SpanElement()
-      ..classes.add('alert')
-      ..style.opacity = '0.75'
-      ..append(text);
-}
+error(text) => createAlert(text, 'alert-error');
+
+warning(text) => createAlert(text);
 
 class CodeCompletionDecoration extends Decoration {
   const CodeCompletionDecoration(
       {String color: '#000000',
        bool bold: false,
        bool italic: false,
        bool stress: false,
        bool important: false})
       : super(color: color, bold: bold, italic: italic, stress: stress,
               important: important);
 
   static from(Decoration decoration) {
     return new CodeCompletionDecoration(
         color: decoration.color,
         bold: decoration.bold,
         italic: decoration.italic,
         stress: decoration.stress,
         important: decoration.important);
   }
 
   Element applyTo(text) {
     var codeCompletion = new DivElement()
         ..contentEditable = 'false'
         ..classes.add('dart-code-completion');
     return super.applyTo(text)
         ..classes.add('dart-code-completion-holder')
         ..nodes.add(codeCompletion);
   }
 }