This test was outputting the "success" string multiple times, which is probably causing the harness to kill the app halfway through its cycle. I suspect this is causing some of the flakiness we've seen of this test. Instead, we should just output the string at the very end of the test, right before the app is done.
* Adds support for `flutter test --wasm`.
* The test compilation flow is a bit different now, so that it supports compilers other than DDC. Specifically, when we run a set of unit tests, we generate a "switchboard" main function that imports each unit test and runs the main function for a specific one based off of a value set by the JS bootstrapping code. This way, there is one compile step and the same compile output is invoked for each unit test file.
* Also, removes all references to `dart:html` from flutter/flutter.
* Adds CI steps for running the framework unit tests with dart2wasm+skwasm
* These steps are marked as `bringup: true`, so we don't know what kind of failures they will result in. Any failures they have will not block the tree at all yet while we're still in `bringup: true`. Once this PR is merged, I plan on looking at any failures and either fixing them or disabling them so we can get these CI steps running on presubmit.
This fixes https://github.com/flutter/flutter/issues/126692
