Just noticed that reduce_vars doesn't respect top_retain for regular (non function) variables...

$ echo 'var x=1, y=2; console.log(x, y, x+y);' | terser -c top_retain=y
var y=2;console.log(1,2,3);

PR hasn't addressed that yet.

Okay, this PR now has top_retain handling functions, classes and regular variables correctly with reduce_vars. Added extensive tests including variations with classes, let, const and arrow functions. Risk and performance impact to default minify should be minimal as all new logic is gated behind compressor.top_retain.

Will leave as WIP for a bit in the event some other broken scenario comes up.

This PR is good to merge.

Also added source map support to test/run-tests.js because errors in ill-formed tests were otherwise completely undecipherable.

@kzc Thank you! Verified the #161 tests are passing with the current version of this PR.

@fabiosantoscode Feel free to merge.

Between this PR and #166 it's probably worthy of a release.

Published 3.10.12