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

dart/site/try/src/shadow_root.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.shadow_root;
+
+import 'dart:html';
+
+void setShadowRoot(Element node, text) {
+  if (text is String) {
+    text = new Text(text);
+  }
+  getShadowRoot(node)
+      ..nodes.clear()
+      ..append(text);
+}
+
+
+/* ShadowRoot or Element */ getShadowRoot(Element node) {
+  if (ShadowRoot.supported) {
+    ShadowRoot root = node.shadowRoot;
+    return root != null ? root : node.createShadowRoot();
+  } else {
+    Element root = node.querySelector('[try-dart-shadow-root]');
+    if (root == null) {
+      root = new SpanElement()
+          ..setAttribute('try-dart-shadow-root', '');
+      node.append(root);
+    }
+    return root;
+  }
+}
+
+void removeShadowRootPolyfill(Element root) {
+  if (!ShadowRoot.supported) {
+    List<Node> polyfill = root.querySelectorAll('[try-dart-shadow-root]');
+    for (Element element in polyfill) {
+      element.remove();
+    }
+  }
+}