Description

Firefox 62 and Chrome 69 introduce the flat function to array object mdn.
This breaks MapSore2 because of in layers object we use flat name property.
We have to check and fix all the state.layers.flat statements.

In case of Bug (otherwise remove this paragraph)

Browser Affected
(use this site: https://www.whatsmybrowser.org/ for non expert users)

• Internet Explorer
• Chrome
• Firefox
• Safari

Browser Version Affected
Firefox from version 62 and Chrome from version 69

Steps to reproduce

• MS2 don't work with Firefox 62
• Travis build fails

Expected Result

• MS2 works with Firefox 62
• Travis build success

Current Result

• Describe here the current behavior

Other useful information (optional):