custom-upload is a wrapper for <vaadin-upload> web component to allow marking an uploaded file as invalid if requirements on the uploaded file are not fulfilled.
Install the Polymer-CLI
First, make sure you have the Polymer CLI installed. Then run
polymer serve to serve your element locally.
Viewing Your Element
$ polymer serve
$ polymer test
Your application is already set up to be tested via web-component-tester. Run
polymer test to run your application's test suite locally.