Bug 953296 - Implement mozilla::RemoveExtent.  r=jcranmer

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extra : rebase_source : 97d4f055fc80074265bd5c8b1b0a565d8e994e7b
Bug 953296 - Add IsLvalueReference and IsVoid.  r=jcranmer

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extra : rebase_source : 53def7d45901da43ffbcfef3adbbea3a3e6bb329
Bug 1027693 - Add a test for top-level let not creating a variable that'd show up in |with (global)| scope.  r=jorendorff over IRC on original landing, minor adjustments to eliminate a log-highlighting false positive r=trivial

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extra : rebase_source : a43dd2daa969aafbab9d1354c82e2a65ffbdd907