I thought classes were safe :(

unsafeClass.js

'use strict';
/*@flow*/

class Square {
    /*::
    x: number;
    y: number;
    w: number;
    */
    constructor (x/*: number*/, y/*: number*/, w/*: number*/) {
        this.x = x;
        this.y = y;
        this.w = w;
    }
}

const area
    /*: (square: Square) => number*/
    = (square) => {
        if (!(square instanceof Square)) {
            throw new TypeError('oops, not a Square!');
        }
        return square.w * square.w;
    };

// This type-checks and runs without error
area(new Square(1, 2, 6));

// This typechecks and throws a TypeError.
area ({ x:1, y:2, w:6, foo:'bar' });