Test for super() returning object other than `this`

[justinfagnani]

If a constructor returns a value other than this, then in subclasses super() calls to that constructor should use the result as this in the subclass constructor.

This is specified in 12.3.5.1, step 10 of SuperCall: http://www.ecma-international.org/ecma-262/6.0/index.html#sec-super-keyword

Here's an example that works in correct ES2015 implementation, and fails in some others:

class Foo {
  constructor() {
    return {
      x: 1,
    };
  }
}

class Bar extends Foo {

  constructor() {
    super();
    this.y = 2;
  }
}

let o = new Bar();
console.assert(o.x === 1 && y === 2);

[nilssolanki]

console.assert(o.x === 1 && o.y === 2);