start talking about polymer.dart

src/site/articles/web-ui/spec.markdown

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 layout: default
 title: "Web UI Specification"
 rel:
   author: siggi-cherem
 description: "A detailed reference guide of how to use Web UI for declarative modern web apps."
 has-permalinks: true
 ---
 {% comment %}
 
 See README comment in index.markdown for explanations on {{'{{'}}, \{\% raw \%\}
 and \{\% codesample \%\}.
 
 {% endcomment %}
 
 # {{ page.title }}
 {: .no_toc}
 
+<aside class="alert alert-danger" markdown="1">
+<strong>Web UI is now Polymer.dart.</strong>
+Web UI has evolved to
+[Polymer.dart](/polymer-dart/), which provides many fixes and improvements.
+We encourage Web UI users to upgrade to Polymer.dart.
+The information on this page apply only to Web UI, which is now deprecated.
+</aside>
+
 _Written by Sigmund Cherem<br />
 November 2012 (Updated February 2013)_
 
 The Web UI package (Web UI for short) provides web components and templating to
 help you write web applications at scale. This article contains a detailed
 specification of Web UI's features. For a quick reference table of
 these features, please refer to our [summary article](summary.html); for a
 high-level introduction and examples, see our
 [explainer article](/articles/web-ui/).
 
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 [dart-scope-rules]: http://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.jb82efuudrc5
 [project]: https://github.com/dart-lang/web-ui/
 [mdv]: http://code.google.com/p/mdv/
 [sd]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html
 [shadow-el]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#shadow-element
 [wc]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html
 [wcappendix]: http://dvcs.w3.org/hg/webcomponents/raw-file/tip/explainer/index.html#appendix-b-html-elements
 [tostring]: http://api.dartlang.org/docs/bleeding_edge/dart_core/Object.html#toString
 [safehtml]: https://github.com/dart-lang/web-ui/blob/master/lib/safe_html.dart
 [elemevents]: http://api.dartlang.org/docs/releases/latest/dart_html/Element.html