Flow template editor fails

When I open with "Flow template editor" a html file of the atarted pack for vaadin 10 "Full Stack App with Spring" I get the error:

Unresolved imports

I found in documentation : “…your project must have Polymer dependency placed inside webapp folder …” but where you found this files to be placed there?

The app it runs fine, but fails the Designer Editor.

A flow chart can help set you straight. A flow chart is a diagram that visualizes a process or workflow. Typically, you uses boxes or shapes to represent different steps in a process, and then you connect those steps with lines or arrows. That’s why it’s a good idea to start with a flow chart template. [Tell Dunkin]

Sorry I don’t understand what is the relation of FLow Char and the issue I post. I just install “Full Stack App with Spring” and when I try to edit a HTML file, with the Flow template editor, I get the indicate error (Unresolve Imports…)