TypeScript:unions

Build 17238 on 8342-dynamic-source
Reviewing disabled
Baseline History
Build 17238acb98c8
Current build. There are unaccepted changes, so build 17181 is still the baseline.
Build 17161d48a9c9
Accepted by Dominic Nguyen.
Build 171055b1f2f7
Accepted by Michael Shilman.
Build 1702520cfdda
Accepted by Norbert de Langen.
Commenting is disabled because there's a newer build on this branch