b3b2a1cebf
This requires some tweaks to to/from JSON code, and some handling for Maps that previously could be serialised directly but now may contain `Uri`s in keys and need to be converted to strings explicitly (since Uri has no toJson method). Change-Id: I61358d8198ac1da322fae98d6c40747ad08754b7 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/258927 Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>