Update NodeListImp constructors, owner, initialize.

Both Brian and Paul noted that in general we should express the
semantics correctly, if something is expected to be not null before
it is used, it would be nice to type it as such. This change makes
`_owner` late final, so that it is always not null, and set only once.
It should not affect performance negatively because almost always
we use `owner` inside `_initialize`, where it is a local variable.

Change-Id: I2abe45f2f98e9ac0ce69d4a65d641334244876ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/183044
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov