Do not produce an error when encountering a new type in a service response. (#42972)

Flutter tool's service client library tries to "upgrade" sub-components of service responses with higher level model objects. It does this for all members of the response, rather than only those documented in the version it targets, and it errors out when encountering a new type. This causes it to fail when a new field is introduced with a new typed. Clients are required to ignore this.