Prevent getting of unknown properties via ES6 proxies

PreventUnknownGet.js

// The following code is valid ECMAScript 6, but doesn’t work in Firefox, yet

function PreventUnknownGet() {
}
PreventUnknownGet.prototype = new Proxy(Object.prototype, {
    get(target, propertyKey, receiver) {
        if (!(propertyKey in target)) {
            throw new TypeError('Unknown property: '+propertyKey);
        }
        // Make sure we don’t block access to Object.prototype
        return Reflect.get(target, propertyKey, receiver);
    }
});
class Point extends PreventUnknownGet {
    constructor(x, y) {
        this.x = x;
        this.y = y;
    }
}

var p = new Point(5, 7);
console.log(p.x); // 5
console.log(p.z); // TypeError

// Alternative: wrap proxy around `this`
// Prevent _creation_ of unknown properties via non-extensibility