webpack: [4] Webpack turns "this" to undefined in class definitions

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

See https://github.com/webpack/webpack/issues/6357#issuecomment-360308225 and https://github.com/babel/babel/issues/7264

Webpack turns the method in this code:

import React from 'react'

class A {
    constructor() {
        this.b = 1
    }
    method(a = this.b) {
        console.log(a)
    }
}

into

    method(a = undefined.b) {
        console.log(a)
    }

probably confusing the top-level with class-level. Curiously without the import (any import) it seems to be fine.

Please mention other relevant information such as the browser version, Node.js version, webpack version and Operating System.

MacOS High Sierra, Node 9.4.0, Webpack 4 beta.0