BazelCI broken on ubuntu 18.04

[c-parsons]

With 28d48b0, I believe we raised the ubuntu CI machine config from 16.04 to 18.04

Unfortunately, I don't think CI has ever passed on 18.04.

Example breakage:
https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/2091#ad491a93-0152-4a50-a363-876f1318cb59

WARNING: java.io.IOException: No such file or directory
--
  | Compiling module io.bazel.sample.SampleApp
  | [ERROR] Hint: Check that your module inherits 'com.google.gwt.core.Core' either directly or indirectly (most often by inheriting module 'com.google.gwt.user.User'

Maybe something related to java version?

[c-parsons]

@philwo

[philwo]

I think rules_gwt will have to update its GWT version to 2.9.0, because the currently used one (2.8.0?) is not yet compatible with Java 11.

I think it's defined here: https://github.com/bazelbuild/rules_gwt/blob/master/gwt/gwt.bzl

Backstory:

• Don't test on Ubuntu 18.04, because it's broken. #24
• Java 11 Compatibility check: GWT gwtproject/gwt#9626

[Wyverald]

Sent PR #38