Added landing pages for new samples

src/site/samples/dndfiles/index.markdown

Index: src/site/samples/dndfiles/index.markdown
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+---
+layout: default
+title: Working with the Files API
+live_example_url: example/index.html
+
+header:
+  css: ["/samples/samples.css"]
+---
+
+## {{ page.title }}
+
+A Dart port of the Files API examples used in the
+[Reading files in JavaScript using the File APIs](http://www.html5rocks.com/en/tutorials/file/dndfiles/)
+article by Eric Bidelman, originally published on HTML5Rocks.
+
+The examples show how to select a file using a form input, or using drag and
+drop. See how you can slice a file in separate byte range
+chunks for efficient uploading, and monitor progress while uploading a file.
+
+Read the
+[source](https://github.com/dart-lang/dart-samples/tree/master/web/html5/file/dndfiles).
+
+<iframe class="running-app-frame"
+        style="height:500px;width:100%;"
+        src="{{page.live_example_url}}">
+</iframe>
+
+See all [samples](/samples/).