Support ES6 method-shorthand constructors in extend (#4213)

[BigBalli]

Fixes #4213.

Problem

ES6 method shorthand (constructor() {}) produces a non-constructable
function with no prototype property. Passing such a constructor to
extend previously caused new Subclass() to throw
Subclass is not a constructor:

const MyModel = Backbone.Model.extend({
  constructor() {
    Backbone.Model.apply(this, arguments);
  }
});
new MyModel(); // TypeError: MyModel is not a constructor

This forces users on modern JS to write the awkward mixed form
(constructor: function() {} next to method shorthand) or to give up on
extend altogether.

Fix

Detect the shorthand case by checking for the absence of prototype on
the supplied constructor, and in that case wrap it in a thin function so
new continues to work. The traditional constructor: function() {}
form is unchanged and still uses the supplied function directly, so the
edge case noted in the issue (Model === proto.constructor) remains
true for ES5 users.

Test

Added a regression test in test/model.js that defines the
constructor via ES6 method shorthand (constructed via eval so the
test file still parses on engines without shorthand support) and
verifies that new Model({foo: 'bar'}) returns a usable instance.

npm run lint passes.