Changes fields to be either mutable or immutable by construction.
This ensure that we don't create setter references for fields that
cannot be assigned to and is a prerequisite for replacing @fields/
@fields= canonical names with @getters/@setters.
TEST=existing expectation tests and verification
Change-Id: I70b9a504ee6f221b7c334ac02620feb0d5f7ae01
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/176665
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Dmitry Stefantsov <dmitryas@google.com>
Johnni Winther
60702d4158