Fix build error caused by upstream change from uglify-js (#251)

[lettenj61]

This patch was originally written for elm-jp/guide by @negiboudu san (ref)

[David-Klemenc]

Hi, I remembered coming across this issue a while ago and several PR are in progress to solve it. I think this issue spans multiple repositories and has conflicting PR:

Issues:
#251
elm-lang 849
elm-lang 848

PRs:
compiler (solution with terser)
#240
and this one

I'm using terser, the main advantage is that it only requires one pass:
terser $js --compress "pure_funcs=[F2,F3,F4,F5,F6,F7,F8,F9,A2,A3,A4,A5,A6,A7,A8,A9],pure_getters,keep_fargs=false,unsafe_comps,unsafe" --mangle --output=$min

I think there should be a discussion (maybe on discord?, maybe here?) to solve this reoccurring problem.

[evancz]

I believe this is fixed by #258 as well. Just merged that one!