Nodes: Remove polyfill for object rest/spread #4876
Description
The object rest/spread syntax, e.g. { x, y, ...z }, has reached Stage 4 (a.k.a. it’s been approved as part of ES2018). When this feature was implemented in #4493, the syntax was only Stage 3, and so a polyfill was added as per our policy. But now that the syntax is finalized, and supported by Node 8.6+, we can remove the polyfill.