Transform frozendicts to dicts to allow serialization.

Frozen dicts are failing to serialize making several builders to fail
with infrastructure issues.

Change-Id: I4e697292a0a4983db5d3d921d1d34fc06441d1fe
Bug: https://github.com/flutter/flutter/issues/65223
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6321
Reviewed-by: Tong Wu <wutong@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>