Added dart:js samples landing pages.

src/site/samples/jsonp/index.markdown

Index: src/site/samples/jsonp/index.markdown
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+---
+layout: default
+title: Fetching Data Using JSONP
+live_example_url: example/index.html
+
+header:
+  css: ["/samples/samples.css"]
+---
+
+## {{ page.title }}
+
+This sample uses `JSONP` and the `dart:js` library to retrieve data about
+Github repos belonging to the user dart-lang.
+
+Read the
+[source](https://code.google.com/p/dart/source/browse/#svn%2Fbranches%2Fbleeding_edge%2Fdart%2Fsamples%2Fjsonp).
+
+<iframe class="running-app-frame"
+        style="height:500px;width:100%;"
+        src="{{page.live_example_url}}">
+</iframe>
+
+See all [samples](/samples/)