Your example could work if you upscale the coordinates before AddPolygons call and downscale them after union operation, eg. by 1000000000000. The scaling is needed to maintain precision, when floats are used, because Clipper uses integers innerly. This has a downside that the operation is slow, because Clipper uses big integer library, if coordinates are upscaled heavily.
To overcome the slowness (and precision) issue, there is available a new (though still experimental) "floats" Clipper: http://jsclipper.sourceforge.net/6.1.3.1_fpoint/clipper.js http://jsclipper.sourceforge.net/6.1.3.1_fpoint/clipper_unminified.js
Clipper 6 has some things changed, which you have to take into account when migrating from Clipper 5 to 6: https://sourceforge.net/p/jsclipper/wiki/migration5to6/
I made a change for you: http://jsfiddle.net/uF6ec/2/
function dummy(){}