Bug 948583 - Implement __proto__ in object literals using a new JSOp.  The JSOp still uses setprop-like behavior to mutate the [[Prototype]]; we'll change that in a subsequent patch, coming probably in the next couple weeks (definitely before uplift).  r=jorendorff, r=efaust

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extra : rebase_source : d779071661bbb1375b8eff60afa7cdf36d2c3005