Add a test for top-level let not creating a variable that'd show up in |with (global)| scope.  No bug, r=jorendorff over IRC

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extra : rebase_source : bd068aeb3021789ec13d2274f72fa728f6d5c4d5
Bug 1020034.  r=bz

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extra : rebase_source : a38cdad564a8499b28f2f83dd1ad63f762419383