…ate protos (#869)

- Rename a local variable to avoid proto field name conflicts.

  This fixes an issue we're seeing with the latest version of
  `package:protoc_plugin` and the dartpad proto definitions.

- Re-generate descriptor protos.

- Update CI config to re-run publishing when labels change.

We used to add a prefix before, but when I brought back the constructor
arguments I didn't include it because (1) the previous prefix was `_` which
caused lint errors (2) there were no tests or documentation on why the prefix
was needed.

Document it now so that it won't be removed again.

(I couldn't add this in #869 as I didn't have push access to the PR's repo)

This updates benchmark builder with two dart2wasm targets:

- wasm: Default optimized build.
- wasm-omit-checks: Optimized build with `--omit-checks`.

README is updated with instructions to build and run new targets.