This is a plugin for the Babel transpiler that polyfills Object.setPrototypeOf and Object.getPrototypeOf in IE9 and IE10. Otherwise static member inheritance of classes and super calls will break.
While this plugin is spec comliant than loose mode the following will not work as expected in regards to Object.setPrototypeOf:
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Properties that are added to the new prototype after Object.setPrototypeOf was called will not be reflected.
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Any use of the delete keyword on any inherited property will also remove the inheritance for that property. So if you overwrite and inherited property and delete it again there will be no fallback to the prototype.