Extending String issue

[normalser]

If this is duplicate of #12123 then please close it

"use strict"
class Dupa extends String {
    constructor(data) {
        super(data)
        this['label'] = data.value
    }
}

const dupa = new Dupa({value: 'Test'})
console.log(dupa['label'])

Works in:

• Chrome
• Firefox
• Node: 6.7.0
• Babel: https://goo.gl/eo6QyN

TS compiled code causes runtime error:
VM2008:11 Uncaught TypeError: Cannot create property 'label' on string '[object Object]'(…)

[mhegazy]

this indeed is a duplicate of #12123, please see the documentation in https://github.com/Microsoft/TypeScript-wiki/blob/master/Breaking-Changes.md#extending-built-ins-like-error-array-and-map-may-no-longer-work