b272632450498f1c688fe4b0988fa595a0004124 exposed client certificate through 
`X509Certificate? get clientCertificate;` getter on `ServiceCall` class. This broke 
compilation of `grpc_web` code using DDC, but not dart2js. Turns out that dart2js is 
happy to compile any code using `dart:io` (though the result will not run if you try
to use any of those APIs), but DDC rejects such code eagerly. `package:test` runs 
tests through `dart2js` so DDC breakage was not really caught by CI. 

Unfortunately this discrepancy between DDC and dart2js puts us in some really weird 
spot: most of our tests are platform independent, but most of those tests also 
pull in `dart:io` through transitive dependencies.

This commit is the most minimal change we could make to allow the code compile both 
on the Web and natively. 

A proper fix should be to go through tests one-by-one and make sure that those that
need to run on the Web don't import `dart:io`, but we don't have time to do that 
right now. 

This commit also adds a smoke test to the CI to verify that `grpc_web` example 
builds with DDC.