[w3c/webcomponents] 'attributeChangedCallback' not working with a data attribute on Firefox or Edge (#637)

I've created a custom element with the 'attributeChangedCallback' function and added a attribute to the 'observedAttributes'.

The component is ran, however when changing the data attribute, the callback does not fire which it does for none data attributes in Firefox and Edge.  This also works in Chrome and Safari.

The issue was spotted in both Firefox 52.0.2 (64-bit) and 53.0 (64-bit) (Mac), this previously used to work however I cannot get the version it was working in.  It was also spotted in Microsoft Edge 38.14393.0.0

Please find the example code below and observe the console.  (the version of webcomponent polyfill does not make a difference, I've tried them between  1.0.0-rc.1 and 1.0.0-rc.8)

https://jsfiddle.net/t8rbucvd/7/


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https://github.com/w3c/webcomponents/issues/637