Maps broken on IE11

[LeeDr]

Kibana version: 6.6.0 BC2

Elasticsearch version: 6.6.0 BC2

Server OS version: Windows 2012 Server

Browser version: IE

Browser OS version: Windows 10

Original install method (e.g. download page, yum, from source, etc.): zip files

Describe the bug:
Coordinate maps don't show the maps. This is a regression vs 6.5.2 where they work in IE11.

Maybe related to #26412

Steps to reproduce:

1. We may not be loading plugins from src/core_plugins any more?
2. When creating a coordinate map, I don't see any requests for the map tiles. And they don't appear.

Expected behavior: Maps must work

Screenshots (if relevant):

Errors in browser console (if relevant): none

Provide logs and/or server output (if relevant):

Any additional context:

[LeeDr]

This could be a duplicate or overlapping #17613