Add unit tests for assignment and invalid field access.

Change-Id: I8b755ae0078d6353e24834cd15603091d681114c
Bug: skia:10766
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319698
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
10 files changed