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 : 50e9ec9c7a97ec83f597fd07f3db57fc80c604c3
Add comments to js/src/tests/browser.js and js/src/tests/*/shell.js indicating how JS versioning requirements for tests are enforced.  No bug, r=me

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extra : rebase_source : f65153d60d87c117e339f2fd7b550709c4b2a02b