Update the demo to work in IE11

[joshlory]

I don't think we'll want to merge this until we also ship the unbuild files to npm. Otherwise we'll double-include the babel-polyfill when importing the built dist/main.js in the main repo.

[codecov]

Codecov Report

Merging #557 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #557      +/-   ##
==========================================
- Coverage    75.8%   75.79%   -0.02%     
==========================================
  Files          43       43              
  Lines        5378     5378              
==========================================
- Hits         4077     4076       -1     
- Misses       1301     1302       +1

Impacted Files	Coverage Δ
src/js/game/LevelMVC/LevelView.js	84.69% <0%> (-0.09%)	⬇️

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