Since there is no updated JDT in GWT, it might be that you can not use var keyword in lambda parameter definitions for now. Have you tried that? Other than that Java 10 and 11 are pretty much the same in terms of syntax.
I have said it on Gitter recently and I think also here in this group: IMHO GWT should provide monthly / quarterly SNAPSHOT builds with a fixed version number and release them on maven central. These automated releases are only code tested just like SNAPSHOT builds. I could imagine something like
2.8.2 - fully tested
2.9.0 - fully tested
2.9.0-14-g7940d679a - automated release which is based on commit 7940d679a and is 14 commits newer than 2.9.0
2.9.0-25-g8941d679b - automated release which is based on commit 8941d679b and is 25 commits newer than 2.9.0
2.9.1 - fully tested
Alternatively just a timestamp could be used for automated releases between fully tested releases, e.g. 2.9.0-20190103. Personally I like the first version more as I can see how many commits I am missing by comparing the version string.
Would your company be fine with such a solution?