Get framework tests running with dart2wasm #126692

Right now we have the following test coverage for the experimental dart2wasm feature:

• Engine unit tests compile to dart2wasm and run on CI, which extensively exercise the web engine, but does so before the sdk_rewriter transformations
• End-to-end integration tests in the framework that ensure that we can build a flutter app with flutter build web --wasm with different parameters. This doesn't actually execute anything in the app, however

We need to add support for running the framework tests themselves with a dart2wasm compiled app.

One caveat is that, for complicated build reasons, when the dart monorepo runs the framework tests, it won't have wasm-opt in the dart-sdk bundle it is using. So we probably need to just turn wasm-opt off in whatever code paths the dart monorepo may end up using.