Follow-up 2 to bug 1041631, part 1 - Make one last test work when Symbol is not defined. Backported from rev 74637aa07226. a=testonly.

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extra : rebase_source : 980090c510cec767980243b4857e731dbc1e92c5
Bug 1041631, part 2 - Make ES6 Symbols Nightly-only for now. r=Waldo, a=sledru.

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extra : rebase_source : f5bec7e3a6b7efa247897aaba04968e38dcef5bd
extra : amend_source : 666791254599be1c1b1ac0775ad2928c7e4e617d