TypeError: console.warn is not a function (ExecJS::ProgramError)

[heaven]

When optimizing a CSS file.

And csso.js contains this line:

case null:console.warn("[csstree-match] BREAK after "+os+" iterations")

The warn function is missing here:

console = {
  log: function(){},
  error: function(txt){
    throw txt;
  }
};