Instance __proto__ property should exist and be writable.

Although having a mutable __proto__ property doesn't make for a consensus (https://bugzilla.mozilla.org/show_bug.cgi?id=642500) the analysis of node, npm and top 20 most depended upon modules (plus associated dependencies and testing frameworks) shows that the writable __proto__ property is used by JS developers.

The following 5 patterns are quite common and not supported by nashorn.

1) CRITICAL: Update prototype of an Object instance or a function:
 - function updateProto(a) { a.__proto__ = aNewProto; }

2) MAJOR: Class inheritency :
 - DaughterClass.prototype.__proto__ = MotherClass.prototype;

3) MAJOR: Provide a prototype to an Object instance at creation time:
  - var a = { __proto__: aProto, myprop: 9, … };
  - var a = new A(); a.__proto__ = aProto;

4) MINOR : Access to __proto__ reference:
 - x.__proto__ === y.__proto__
 - x.__proto__.constructor

5) MINOR: Provide a prototype to a Function at creation time:
 - var f = function() {}; f.__proto__ = aNewProto;

Usage in most depended upon node modules
====================================
1 occurs in 4 modules: proto-list, express, qs, ejs modules
2 occurs in 15 modules: socket-io, commander, connect, connect-redis, engine-io, send, stylus, browser-builtins, supertest, superagent, emitter-component, mocha, jade, node-unit, cli-table modules
3 occurs in 4 modules: traverse, hashish, express, esprima module test dependency modules
4 occurs in 4 modules: traverse module test script, readable-stream module test script, jade, insert-module-globals modules
5 occurs in 1 module: engine.io

Workaround and side effects
======================
1 can't be workarounded. Script needs to be rewritten in order to propagate prototype updates.

2 can be workarounded with Object.create but DaughterClass prototype can't be updated once Object.create has been called. Furthermore, any instance created prior to the extension are not updated.
DaughterClass.prototype = Object.create(MotherClass.prototype);
DaughterClass.prototype.constructor = DaughterClass;

3 can be workarounded with Object.create
Object.create(aProto, { myprop : {enumerable: true, value:9, ...} }

4 can be workarounded with Object.getPrototypeOf
Object.getPrototypeOf(x) === Object.getPrototypeOf(y)

5 can be workarounded with:
for(k in aNewProto) {
    f[k] = aNewProto[k];
}