[experiment] Try building against java 12

[iamdanfox]

Internally, we're pushing all our services to run on Java11 only as it's a nice LTS release.

For libraries however, we want to keep testing against Java8 to make sure we don't break backwards compatibility. Given we have to keep the java 8 CI, I think we might as well start testing against Java 12, so hopefully we can catch any tooling gotchas early.

cc @robert3005

[iamdanfox]

Seems like it's a known issue that error-prone 2.3.3 doesn't work with Java 12: google/error-prone#1106

[iamdanfox]

I actually just tried this on a much larger internal project (SMM) and errorprone passed nicely when I turned off this single rule!

[gm2211]

Adding to this, another large internal project (ICS server) has source 11 and compiling with jdk11 is super slow, but it's really fast with jdk12 (think 3+ minutes in jdk11 and < 20 seconds in jdk12). Can use jdk12 fine in intellij, but it fails in gradle with:

     Please report this at https://github.com/google/error-prone/issues/new and include the following:

     error-prone version: 2.3.3
     BugPattern: Finally
     Stack Trace:
     java.lang.NoSuchFieldError: label