This is "not yet a standard" and "not even have brought to TC39" at the time of writing (Mar 2017, still stucked at stage 2 https://github.com/tc39/proposals) , how came it was here 1 years ago stating that is part of es2016?
Don't even understand why people are so possessed with new class syntactic sugar. fundamentally it is not CLASS but still PROTOTYPE based, that is why making the property declared inside the class is not considered at the beginning because it causes more confusion then benefits.(methods are in prototype while properties are on each instance)
Not to mention the class syntax (that without the need of assignment) make it looks like is hoisting but in fact it is not and also quite confusing as well.