Add flutter engine html_wasm tests to monorepo CI

[whesse]

The flutter engine test suite chrome-dart2wasm-html-engine is important for finding new failures
in dart2wasm which will block the roll of Dart SDK into Flutter engine.

Add this to the https://dart.googlesource.com/monorepo/+/refs/heads/main/ci/builders/web_linux.json
configuration in the monorepo repository, so it is run on commits to the SDK and engine.

This was requested after the suite failed, causing issue flutter/flutter#137120 and a rollback.

[whesse]

This engine test suite chrome-dart2wasm-html-engine is run as part of the web_linux build/test configuration, by a tester called "Linux run chrome-dart2wasm-html-engine suite" in the json configuration file https://github.com/flutter/engine/blob/main/ci/builders/linux_web_engine.json

The tester doesn't do much more than run felt with the right parameters, to run the test suite.
But it depends on downloading the necessary Flutter engine checkout and built files from CAS, where they have been uploaded by its build dependencies (builders in the same json configuration of the web_linux builder)

"web_tests/artifacts",
 "web_tests/test_bundles/dart2wasm-html-engine"

But these builders are different from our existing flutter-web builders. Monorepo's flutter-web builders upload the artifacts to GCS, so Flutter framework checkouts can run framework tests by downloading the artifacts using flutter doctor.

The new builders listed above do a build and upload their entire output directory to CAS, and the engine testers download it from CAS. The builders listed above also depend on using a prebuilt Dart SDK, and cannot easily switch to building the Dart SDK. Especially the second one, "web_tests/test_bundles/dart2wasm-html-engine", just checks out the engine source and then runs a simple dart compile command, using the dart2wasm or dart2js already downloaded in the prebuilt Dart SDK. This builder would be much more expensive if it needed to build the Dart SDK just to run this bundling step.

So figuring out how to run engine web tests on monorepo will need a big look at the entire design of engine testing in the engine_v2 framework, and how the current split of web testing into builders and testers on Flutter engine CI can be adapted to run on monorepo CI, without a prebuilt SDK. Adding a single web engine tester can't be done without tackling the whole framework.

@eyebrowsoffire @askeksa

An example run of this tester is at https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20Production%20Engine%20Drone/457572

The run of the web_linux builder that runs and collects this subbuild is at
https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20linux_web_engine/5162

[osa1]

It looks like this can't be implemented. Should we close this @mkustermann?

[mkustermann]

We may still want to have HHH setup with dart2wasm, otherwise we may land CLs that break flutter and will only know when rolls happen. There's a question around how this would work, because the flutter-web downloads a prebuilt Dart SDK rather than building it.

Though this isn't very urgent.