Failures on Update ListMixin/ListBase documentation and `insert` for Null Safety.

[lrhn]

There are new test failures on Update ListMixin/ListBase documentation and insert for Null Safety..

The tests

service/step_through_mixin_from_sdk_test/dds RuntimeError (expected Pass)
service/step_through_mixin_from_sdk_test/service RuntimeError (expected Pass)

are failing on configurations

dartk-linux-release-x64

The only changes to the SDK are line numbers. The test appears to assume that those are fixed. It should be changed to not assume that.

[alexmarkov]

/cc @bkonyi