Updating the samples page to reflect the examples that have been retired.

src/site/samples/todo/index.markdown

----
-layout: default
-title: Adding Elements to the DOM
-live_example_url: /docs/tutorials/add-elements/examples/todo/todo.html
-header:
-  css: ["/samples/samples.css"]
----
-
-## {{ page.title }}
-
-Enter text in the input field and press return.
-
-This app shows how to programmatically add an element to the DOM.
-
-In this example, when you press return, the event handler for the input
-field creates a new <li> element and adds it to an existing <ul> list in the
-DOM tree. When the DOM tree changes, the web page refreshes.
-
-Read this [tutorial](/docs/tutorials/add-elements/) for
-more detail, or read the
-[source](https://github.com/dart-lang/dart-tutorials-samples/tree/master/todo).
-
-<iframe class="running-app-frame"
-        style="height:400px;width:100%;"
-        src="{{page.live_example_url}}">
-</iframe>
-
-See all [samples](/samples/).