v12 - avoid needless data copying in fs.promises.writeFileHandle when called with Uint8Array

[joelrbrandt]

Is your feature request related to a problem? Please describe.
In node v12, fs.promises.writeFileHandle performs needless data copying when writing a Uint8Array. See:
https://github.com/nodejs/node/blob/v12.x/lib/internal/fs/promises.js#L151

If data is a Uint8Array, the local variable buffer will actually be a Uint8Array object. Calling .slice on a Uint8Array creates a shallow copy of the values. This is documented both on MDN and in the node documentation.

Describe the solution you'd like
Don't use .slice on Uint8Arrays

Describe alternatives you've considered
Manually making a Buffer out of my Uint8Array before calling fs write functions.

Thanks for all your hard work in making Node awesome!