[dart2wasm] Switch majority of benchmarking to `-O2`

[mkustermann]

As we default now to sound mode in dart2wasm we should add benchmarking of dart2wasm-O2-jsc and dart2wasm-O2-jsshell.

We may still want to keep old dart2wasm-O4-{jsc,jshell} configurations but assign less benchmark resources to it.