The difference is actually somewhat subtle, but is shown here under class/enumerable. Same deal about classes requiring new (a lot of libs actually have bugs for only testing against compiled classes with older Babel and using .apply on classes): https://kangax.github.io/compat-table/es6/
And all I was saying is that the spouting of "TS is a superset of ES6!" is not that simple, since most usage will be (for now) with the ES5 output, where it isn't compliant in subtle ways that do cause problems/bugs. Nothing more :)
Obviously there's even cases here TS is compliant and Babel is not. Babel will call those bugs instead of spouting their compiler has no compromises like a lot of the comments here try to imply to convince people.
Also, properties set on the prototype will fail propertyIsEnumerable() and also hasOwnProperty(). for..in loops should always be filtered by hasOwnProperty() in any sane code base anyway.
(I tested the TS output for that class in Node 5 and my own browser, and the property did not come up in the loop, so I just don't think this is an issue in reality.)