Update tests to avoid getting or setting `.__proto__`. Change-Id: I2e80dfc32f162de4f5b3fe5ac74a9e6818a7e55e Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/317845 Reviewed-by: Srujan Gaddam <srujzs@google.com> Reviewed-by: Sigmund Cherem <sigmund@google.com>