Polyfills not found when running with Undertow

[Artur-]

Starting test-spring-boot-undertow using spring-boot:run outputs

2018-04-06 10:17:52.982  WARN 73294 --- [  XNIO-2 task-1] c.v.f.s.DefaultDeploymentConfiguration   : Unable to locate polyfill webcomponents-loader.js, proceeding without it

The underlying reason is that VaadinServlet.scanForResources is called with path /, which should scan through all webjars but scans through nothing because ServletContext.getResourcePaths("/") returns an empty set on Undertow.

I think that the whole scanForResources is just legacy and should be removed. Instead the polyfill should be found directly using the known path where it should be.