Issue#48 Optionally support object prototypes

[virajsanghvi]

Couple notes:

• I'm still using Object.getPrototypeOf in instantiateEmptyObject because coming from any of the ImmutableObject methods, I wouldn't have options. An alternative, but not as clean, would be to dynamically generate instantiateEmptyObject in makeImmutableObject(), and then reattach it to results in the ImmutableObject methods (probably by passing some options arg to makeImmutableObject()). If you have a better idea, I'm all ears.
• I spent some time trying to generate prototypes with jscheck, but I'm unsure how, and it was hard to find documentation/examples online. Let me know if there's anything you'd like me to do there as the tests are probably lacking, and pointers to docs would be helpful.

[rtfeldman]

Let's inline instantiateEmptyObject into makeImmutableObject so we can just do the prototype check once instead of every time we instantiate an object:

var prototype = Object.getPrototypeOf(this);
var instantiateEmptyObject =
  (prototype === Object.prototype)
    ? instantiatePlainObject
    : (function() { return Object.create(prototype); });

// ... the other addPropertyTo calls go here ... //

addPropertyTo(obj, "instantiateEmptyObject", instantiateEmptyObject);

...and then, elsewhere in the file:

function instantiatePlainObject() { return {}; }

[virajsanghvi]

I want to point out that with the suggested approach, you'd still be doing the prototype check almost every time we instantiate an object. merge and without both call makeImmutableObject on their clones, and would go through this check. The added benefit over the previous approach is that asMutable and merges without anyChanges would not have to go through this check.

One option I was thinking about was passing an options hash to makeImmutableObject with instantiateEmptyObject as an option, which when present, would be used instead of constructing it.

Anyways, let me know if you still want to proceed with the above suggestions.

[rtfeldman]

+1 for passing it as an argument to makeImmutableObject! That sounds like the best approach yet.

[rtfeldman]

This looks great! Two things before merge:

• I had one minor inline code suggestion
• Let's document the prototype option in the README

Other than that, really excellent work! 🎉 Sorry I took so long getting to reviewing it.

[virajsanghvi]

Thanks for your help with adding this!

[maartenvandillen]

I have a question about the use of the prototype param of Immutable. Is there any way I can use this when calling merge? I am trying to merge an object with prototype into my redux store but it loses the prototype after merging.