Support async/await in viewer

[connorjclark]

Requires uglify-es >= 3.0.17. we are on 3.0.15.

Will do after #9546.

[wardpeet]

do we want to move to terser instead? uglify-es isn't activily being worked on. I can pick this up if you like.

[brendankenny]

if it's a drop-in solution SGTM. I mostly don't want to increase complexity since the only goal is a simple minification, and it would be great to be using something being actively developed.

[wardpeet]

Yes, it is. I'll create a PR, feel free to throw it away if it's too complex but it's more or less s/uglify-es/terser

[connorjclark]

terser should do the trick.