infrastructure: check coverage for source instead of generated JS

[ljqx]

We better to check coverage for source file (.ts) instead for the JS file, to

• make it easier to see which part of code is not covered.
• remove generated file fixup https://github.com/jquery/esprima/blob/master/tools/fixupbundle.js

This could be done by source-map and https://www.npmjs.com/package/remap-istanbul.