dist JavaScript throws syntax errors in IE8

[NickColley]

While investigating why the Design System throws errors in IE8 that we don't see in the review application I tried using the built dist and found it fails in IE8.

When copy and pasting https://github.com/alphagov/govuk-frontend/blob/master/dist/govuk-frontend-2.2.0.min.js into JSBin and running it in IE8 browser stack I get the following error:

'prototype' is null or not an object

){n(t.prototype,i,function(){var t,a=this,s="__defineGetter__DEFINE_PROPERTY"+i

Seems to be around the defineProperty polyfill, I'm doing more investigation into why rollup is behaving differently.

The JavaScript compiling seems to be the same, with just the destination changing: https://github.com/alphagov/govuk-frontend/blob/master/tasks/gulp/compile-assets.js#L101

[NickColley]

I believe the review application does not run an uglifier, and the Design System and dist files are both uglified, so this could be the cause.

[NickColley]

In the Design System setting the uglify option ie8: true solved this issue, so likely will here, will do a PR when I get a chance.