Fold another manual copy into `Object.assign`
Fold another manual copy into `Object.assign`

Previously, this field was being assigned twice: once via direct
assignment in the constructor, and once via the `copy` (previously) or
`Object.assign` (now in this stream of changes) call.

tildeio/glimmer,glimmerjs/glimmer-vm,tildeio/glimmer,glimmerjs/glimmer-vm,glimmerjs/glimmer-vm,tildeio/glimmer