Because the cost of type checks dominate our dart2wasm benchmarks, we've
decided to pass `--omit-type-checks` for now.

This was previously reverted because the skwasm benchmarks were broken
in general for a separate reason, and my getting rid of `bringup: true`
broke the tree. I ended up fixing the benchmarks and getting rid of
`bringup: true` in a separate commit, so this just adds the flag only.