[dart2wasm] Redirecting from legacy url is broken for dart2wasm-compiled app

[elliette]

I noticed that when trying to load DevTools using the legacy URL format (e.g. http://127.0.0.1:9102/#/inspector?uri=ws://127.0.0.1:51066/lBG45naB7hc=/ws) it crashes for the dart2wasm-compiled app. For dart2js-compiled app it succeeds.

[elliette]

This is because flutter_bootstrap.js adds the compiler query parameter before the # in the legacy URL format, which is in keeping with standard Web guidelines. However, DevTools expects the query parameters to be after the hash.

There are two places that this could be fixed:

1. In flutter_bootstrap.js, modify the logic to add the compiler query parameter after the hash
2. In the Dart mapLegacyUrl function which takes the legacy URL format and redirects to the new URL format. We could add logic to handle any compiler query parameters before the hash in the legacy format.

I prefer fixing this in mapLegacyUrl so that we can also add tests, which we can't do (or at least not easily) for flutter_bootstrap.js)