Add some test cases involving Symbol.iterator as a prelude to bug 918828. no_r=me, testonly.

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extra : rebase_source : 545cd0c1618b7f453c3afa2baa8eb4d860e7e6d2
extra : amend_source : 71045431741656cedad8832c7c690d4c96434b9a
Bug 918828, part 1 - Add JSAPI macros JS_SYM_FN etc. to support defining functions with well-known symbol keys. r=Waldo.

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extra : rebase_source : 4fc59b62937294fd25564b35da0abebcb7d9c2d3
Bug 918828, part 2 - Change iteration code to call iterable[Symbol.iterator]() rather than iterable["@@iterator"](). r=nbp.

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extra : rebase_source : 529d0cd48cd1993cd8cc648ac385ca12342ecfd1
Bug 918828, part 3 - Update decompiler for JSOP_SYMBOL. This improves error messages when @@iterator is called implicitly. r=Waldo.

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extra : rebase_source : b9edaa7be12bdfde40b38d3f413f10624c4f0dcf
Bug 918828, part 4 - Implement JSOP_SYMBOL in the baseline compiler. r=nbp.

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extra : rebase_source : 6b13d0ec63de22883fc3cd186cf24a9455bd2408