[tests] Fix CompileTime error in implement_rectangle test.
[tests] Fix CompileTime error in implement_rectangle test.

Fixes: https://github.com/dart-lang/sdk/issues/41541
Change-Id: I46f069145990f548848e6efef9e8d4c9c939adf2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144942
Reviewed-by: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>
Reviewed-by: Riley Porter <1b4d70a146dadc0b64cc886a93a8a324abb72aeb@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk