Check for duplicate class element names

[Raul-Sebastian Mihăilă]

In section 15.5.1 from http://exploringjs.com/es6/ch_classes.html it is mentioned that duplicate class element names are not allowed. However both Chrome and Firefox allow:

```js

class C {

  func() {}

  func() {}

}

```

Also I couldn't find this check in the spec.

[Mörre Noseshine]

When you do that you merely overwrite what you did previously. Remember that "class" is just another word for a special kind of use of Javascript's prototype-based inheritance, so that any member functions end up as members on the constructor function's prototype object. You know what happens when you assign to the same object property more than once...

Example:

$ node

> class C { func (a) {} func (b) {} }

[Function: C]

> C.prototype.

C.prototype.__defineGetter__      C.prototype.__defineSetter__      C.prototype.__lookupGetter__      C.prototype.__lookupSetter__

C.prototype.__proto__             C.prototype.constructor           C.prototype.hasOwnProperty        C.prototype.isPrototypeOf

C.prototype.propertyIsEnumerable  C.prototype.toLocaleString        C.prototype.toString              C.prototype.valueOf

C.prototype.func

> C.prototype.func

[Function: func]

> C.prototype.func.toString()

'func(b) {}'