A few ES6 class conversions

[DefinitelyMaybe]

Unsure as to whether the following would be the preference of converting
ArrayCamera.prototype = Object.assign( Object.create( PerspectiveCamera.prototype ), { ... isArrayCamera: true
to
constructor() { ... Object.defineProperty( this, 'isArrayCamera', { value: true } ); }

a simple this.isArrayCamera = true does the above simply enough but should we worry about someone then calling ___.isArrayCamera = false?

[Mugen87]

#18863 (comment)

[DefinitelyMaybe]

Ah! thank you again. This PR had changed a couple of var's to let's. I've undone that change. Progress might be slow but yes, one thing at a time is best.

[DefinitelyMaybe]

oops. missed up the merge commit. Will close request and make a new one.