Reland --omit-type-checks for benchmarks. (#131493)

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.

dev/devicelab/lib/tasks/web_benchmarks.dart

@@ -36,6 +36,7 @@ Future<TaskResult> runWebBenchmark(WebBenchmarkOptions benchmarkOptions) async {
       if (benchmarkOptions.useWasm) ...<String>[
         '--wasm',
         '--wasm-opt=debug',
+        '--omit-type-checks',
       ],
       '--dart-define=FLUTTER_WEB_ENABLE_PROFILING=true',
       '--web-renderer=${benchmarkOptions.webRenderer}',