Object contains 'libraryID' and 'key' properties
This is due to changed array destructuring behavior in Firefox. Previously,
`var [foo, bar] = maybeArrayMaybeFalse()` always worked, leaving foo and bar
undefined if the function returned false. Now (with ES6, I assume), if the
function returns false it results in a "false[Symbol.iterator] is not a
function" error. But `var {libraryID, key} = false` works as expected, leaving
both values undefined, so instead we can just return an object with those
properties from getLibraryAndKeyFromID(). To assign to different variables, use
`var {libraryID, key: parentItemKey} = ...`.
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