update all comments/spec references to ES6 spec

Right now there are a mix of references like // ECMA262: x.y.z.w and // ES6: x.y.z.w in comments around code, and we've directly copied (and converted to comments) the spec algorithms in numerous places.

Some of the ECMA262 references are to the ES5 spec, some to ES6. And ES6 recently changed all its numbering so the ES6 references are often out of date as well.

There are also a number of other more fundamental changes (addition of Symbol in ES6 caused all the ToStrings to become ToPropertyKeys when dealing with properties, and there are a host of other subtle changes.

All these need to be updated (and we need to put a stake in the ground as far as ES6 spec date is concerned and say "we implement ES6 as speced in the <date/version> draft." We can then update when we need to by looking at diffs between different draft versions and making only those changes, but what we have right now is kind of a mess.

Start by inventorying the ECMA262 and ES6 references and the copied specification algorithms throughout the codebase; the issue does not identify specific files or tests. Establish the target ES6 draft date or version, then compare references and algorithms against it. Done means the references and affected property-related operations consistently match that chosen specification.