With #3516 it is now possible for a class to extend null as permitted by ES6. This PR changes the __extends helper emitted for derived classes to properly support null base arguments in ES5 environments:

var __extends = (this && this.__extends) || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
};

Since Object.create is available only in ES5, an exception will be thrown for a null base argument in an ES3 environment. But that was already the case before, so we're no worse off.