Assets included in index.html do not get copied over on Windows

**Bug**

Assets (scripts, css and images) included in the index.html file are not included in the build on Windows.

Package Version: 4.0.1

**Code**

https://github.com/dojo/examples/tree/master/custom-element-menu

**Expected behavior:**

The assets should be copied over.

**Actual behavior:**

On windows, assets are not copied over.
