The experimental modern mode runs the type checking plugin twice, which **occasionally** suffers from a race condition that hangs the build.
This PR fixes type checking to only be run once.
While this test cannot 100% reproduce/capture the race condition, I don't feel strongly about the test case:
- We're planning on eliminating this type checking plugin ASAP (for a faster alternative)
- Modern mode implementation as-is will probably go away with webpack 5
8b72b9e2de