Prepare 3.1.0 release (grpc#585)

Fix compilation with DDC (grpc#516)

b272632 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.

Prepare to release 3.0.1 (grpc#514)

Prepare for stable v3 release (grpc#452)

Release 2.9.0 (grpc#437)

Release 2.8.0 (grpc#399)

Bump SDK constraint and add dependency on fixnum (grpc#377)

Grpc-web Handle empty trailers (grpc#247)

* Grpc-web Handle empty trailers

Prepare release 0.4.1 (grpc#80)

Prepare release 0.4.0 (grpc#65)