I was trying ES6 syntax and find I cannot define prototype property or instance property within class defination, why forbids it?

I was using `MyClass.prototype.prop=1` before, try ES7 by babel compiler as below, still cannot define prototype property.

```
class MyClass{
  prop=1;
  static sProp=1;
}

```

I don't think define instance property is any dangerous, there's 2 cases in my own browser game need prototype property:

1. Subclass instances need to inherit same property value from base class:

```
var Building=function(){...}
Building.prototype.sight=350;
TerranBuilding.CommandCenter=...(CommandCenter extends Building)
TerranBuilding.Barracks=...(Barracks extends Building)

```

So CommandCenter and Barracks will both have same building sight as 350.  

```
new CommandCenter().sight===new Barracks().sight//All buildings have same sight

```

2. Buffer effect override original property and remove buffer  

```
Marine.prototype.speed=20
var unit=new Marine()
unit.speed===20//get unit.__proto__.speed 20
unit.speed=5//Buffer:slow down speed, unit.speed will override unit.__proto__.speed
delete unit.speed//Remove buffer
unit.speed===20//true, speed restore

```

So I think it should add a way to set prototype property instead of forbid it completely, or can you give some other solutions to deal with above 2 cases?