Added landing pages for new samples

src/site/samples/imagefilters/index.markdown

Index: src/site/samples/imagefilters/index.markdown
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+---
+layout: default
+title: Using Image Filters with Canvas
+live_example_url: example/index.html
+
+header:
+  css: ["/samples/samples.css"]
+---
+
+## {{ page.title }}
+
+A port of the examples used in the
+[Image Filters with Canvas](http://www.html5rocks.com/en/tutorials/canvas/imagefilters/)
+article by Ilmari Heikkinen, originally published on HTML5Rocks.
+
+The article describes how to use image filters with an HTML5 Canvas element.
+It explains how to process pixels, run simple filters, and blur,
+sharpen and emboss an image using convolution filters.
+
+You can open the example in Dart Editor and run it by clicking `index.html`.
+
+Read the
+[source](https://github.com/dart-lang/dart-samples/tree/master/web/html5/canvas/imagefilters).
+
+<iframe class="running-app-frame"
+        style="height:500px;width:100%;"
+        src="{{page.live_example_url}}">
+</iframe>
+
+See all [samples](/samples/).