IE11 is broke on 7.x branch

[LeeDr]

Kibana version: 7.x branch (v7.7.0)

Elasticsearch version: 7.x branch (v7.7.0)

Server OS version: Ubuntu 16.04

Browser version: IE11

Browser OS version: Windows 10

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

Describe the bug: IE11 never loads, shows a bunch of errors in the browser console.

Steps to reproduce:

1. git checkout 7.x
2. yarn kbn bootstrap
3. cd x-pack
4. node scripts/functional_tests_server.js
5. open Kibana in IE11

Expected behavior: IE11 is still supported to needs to load Kibana

Screenshots (if relevant):

Errors in browser console (if relevant):

SCRIPT1002: Syntax error
security.plugin.js (58353,1)
SCRIPT5007: Unable to get property 'plugin/security' of undefined or null reference
login (18035,7)
SCRIPT1010: Expected identifier
spaces.plugin.js (25324,14)
SCRIPT5007: Unable to get property 'plugin/spaces' of undefined or null reference
login (18035,7)
SCRIPT1002: Syntax error
data.plugin.js (165799,1)
SCRIPT5007: Unable to get property 'plugin/data' of undefined or null reference
login (18035,7)
SCRIPT1002: Syntax error
alerting.plugin.js (6097,35)
SCRIPT5007: Unable to get property 'plugin/alerting' of undefined or null reference
login (18035,7)
SCRIPT1002: Syntax error
triggers_actions_ui.plugin.js (23255,8)
SCRIPT5007: Unable to get property 'plugin/triggers_actions_ui' of undefined or null reference
login (18035,7)
SCRIPT1002: Syntax error
file_upload.plugin.js (16601,1)
SCRIPT1002: Syntax error
reporting.plugin.js (57998,1)
SCRIPT1002: Syntax error
advancedSettings.plugin.js (58826,1)
SCRIPT1002: Syntax error
navigation.plugin.js (13099,1)
SCRIPT1002: Syntax error
snapshotRestore.plugin.js (165950,1)
SCRIPT5007: Unable to get property 'plugin/file_upload' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT5007: Unable to get property 'plugin/reporting' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT5007: Unable to get property 'plugin/advancedSettings' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT5007: Unable to get property 'plugin/navigation' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT5007: Unable to get property 'plugin/snapshotRestore' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
transform.plugin.js (188934,1)
SCRIPT5007: Unable to get property 'plugin/transform' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
watcher.plugin.js (165950,1)
SCRIPT5007: Unable to get property 'plugin/watcher' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
indexManagement.plugin.js (171682,1)
SCRIPT5007: Unable to get property 'plugin/indexManagement' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
endpoint.plugin.js (62668,1)
SCRIPT5007: Unable to get property 'plugin/endpoint' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
graph.plugin.js (86184,1)
SCRIPT5007: Unable to get property 'plugin/graph' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
dataEnhanced.plugin.js (165799,1)
SCRIPT5007: Unable to get property 'plugin/dataEnhanced' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
dashboard.plugin.js (178314,1)
SCRIPT5007: Unable to get property 'plugin/dashboard' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
ml.plugin.js (210791,1)
SCRIPT5007: Unable to get property 'plugin/ml' of undefined or null reference
commons.bundle.js (18035,7)
SCRIPT1002: Syntax error
infra.plugin.js (239050,1)
SCRIPT5007: Unable to get property 'plugin/infra' of undefined or null reference
commons.bundle.js (18035,7)

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

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[LeeDr]

Looks pretty much the same when I run OSS from kibana/

[spalger]

I'm not having any issue anymore, additionally we finally implemented some protections to avoid this from being an issue in the future with #59972