Run like a normal meteor program, ie. meteor run
Alternate clicking both buttons and eventually the console log will spit out disturbingly large numbers (around 10k). Can take a few tries. Alternatively, running Widgets.update({}, {$set: {a: 5}}, {multi:true}) and Widgets.update({}, {$unset: {a: 1}}, {multi:true}) from a meteor shell seem to give more consistent results.