Fix compilation errors from mock canvas copy

[Piinks]

Follow up PR. The last PR fixed analysis issues, now we are blocked on some compilation issues from copied code we are trying to make public in https://github.com/flutter/flutter/pull/131631

The customer_testing branch and flutter/tests will need to be updated again once this lands.

Pre-launch Checklist

• [x] I have incremented the package version as appropriate and updated CHANGELOG.md with my changes
• [x] I have added/updated relevant documentation
• [x] I have run "optimize/organize imports" on all changed files
• [x] I have addressed all analyzer warnings as best I could

[Piinks]

@GroovinChip 😁

[GroovinChip]

@Piinks actually I just noticed that the last PR didn't update the version in the pubspec, so can we actually remove the 2.0.1 changelog entry too?

[Piinks]

Sure! One sec!

[GroovinChip]

@Piinks customer_testing has been updated!