Failures on change: [ddc] Add non-null assertions when setting fields

[nshahan]

There are new test failures on [ddc] Add non-null assertions when setting fields.

The tests

language/nnbd/null_assertions/parameter_checks_fields_and_setters_test RuntimeError (expected Pass)

are failing on configurations

dart2js-hostasserts-weak-linux-x64-d8

I added some additional tests for the null-assertions flag when I updated DDC to throw when assigning a null value to a non-nullable field when the null-assertions flag is enabled. This was at the request of a google3 customer to aid in the null safety migration.

[fishythefish]

/cc @rakudrama