Bug 1041631, part 1 - Change Symbol tests to pass if Symbol is not defined. r=Waldo.

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extra : rebase_source : b8510807640d3f0da483904079a26467829f7e70
Bug 1041631, part 2 - Make ES6 Symbols Nightly-only for now. r=Waldo.

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extra : rebase_source : f2365f340299edac9db040280a2a13aa254e5148