Added custom tags helper classes to the Observatory

runtime/observatory/lib/src/elements/helpers/tag.dart

Index: runtime/observatory/lib/src/elements/helpers/tag.dart
diff --git a/runtime/observatory/lib/src/elements/helpers/tag.dart b/runtime/observatory/lib/src/elements/helpers/tag.dart
new file mode 100644
index 0000000000000000000000000000000000000000..177e2383a150c7988b43b452808825dbb2dafb1f
--- /dev/null
+++ b/runtime/observatory/lib/src/elements/helpers/tag.dart
@@ -0,0 +1,36 @@
+// Copyright (c) 2016, 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.
+
+import 'dart:html';
+
+/// Utility class for Custom Tags registration.
+class Tag<T extends HtmlElement> {
+  /// Tag name.
+  final String name;
+  /// Dependend tags that need to be registred for this tag to work properly.
+  final Iterable<Tag> dependencies;
+
+  const Tag(this.name, {this.dependencies : const []});
+
+  static final Map<Type, String> _tagByClass = <Type, String>{};
+  static final Map<String, Type> _classByTag = <String, Type>{};
+
+  /// Ensures that the Tag and all the dependencies are registered.
+  void ensureRegistration() {
+    if (!_tagByClass.containsKey(T) && !_classByTag.containsKey(name)) {
+      document.registerElement(name, T);
+      _tagByClass[T] = name;
+      _classByTag[name] = T;
+      dependencies.forEach((tag) => tag.ensureRegistration());
+    }
+    var tag = _tagByClass[T];
+    if (tag != name) {
+      throw new ArgumentError('Class $T is already registered to tag ${tag}');
+    }
+    var c = _classByTag[name];
+    if (c != T) {
+      throw new ArgumentError('Tag $name is already registered by class ${c}');
+    }
+  }
+}