Further split up the dictionary-property-redefinition tests (beyond their current eight-way split, to a thirty-two-way split) so they don't potentially time out so readily.  No bug, r=me as mechanical

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extra : rebase_source : 087012673cd68be6a30542802e84ad326a0b61c6
Bug 937855 - Implement Object.assign.  Patch is a tag-team effort with till.  Also add tests: most courtesy of André Bargull, some from me, some suggested by jwalden.  r=jwalden

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extra : rebase_source : 4dbbb9de2a833769d34e5ad80d041d4e6de9ab18