I think the reason for these is for js minification. With these assignments a simple minifier will replace those three variables with a line like this:

    var a=null,b=true,c=false;

Then your minified js will be smaller because all other instances of true, false and null will be one character. Whether or not this is a reasonable optimization is debatable but in terms of pure code size it would be a win.

That is part of the es6-collections polyfill included as part of smap.js. If I understand correctly, it is for safety (for example you cannot override the value of "true").