Update: Accidentally use `--O4` instead of `-O4` in `dev/devicelab/lib/tasks/web_benchmarks.dart` update.

Original description:

* Make `flutter build web` have one option that determins the
optimization level: `-O<level>` / `--optimization-level=<level>` =>
Defaulting to -O4 => Will apply to both dart2js and dart2wasm

* Deprecate `--dart2js-optimization=O<level>`

* Disentagle concept of optimization from concept of static symbols =>
Add a `--strip-wasm` / `--no-strip-wasm` flag that determins whether
static symbols are kept in the resulting wasm file.

* Remove copy&past'ed code in the tests for wasm build tests

* Cleanup some artifacts code, now that we no longer use `wasm-opt`
inside flutter tools