Bug 1040699 - Remove "group assignment" optimization in destructuring assignment; r=jorendorff

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extra : rebase_source : ffb266f0aace39aa80026bad1c7ba3eb336dfec1
Bug 933276 - Implement [...x] in assignment target (spread operator); r=jorendorff

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extra : rebase_source : e0d6d62d1c405b89037de7dbb98b0aaaec8f35cd