dart2wasm compilation tests

[elliette]

We don't currently have any tests for our wasm-compiled app besides our benchmark tests.

Ideally, I would like to have a test that:

1. serves devtools in release mode
2. modifies the query parameter (to either compiler=wasm or compiler=js)
3. verifies devtools is now using the wasm or js compilation
4. verifies the local storage file containing the users preference now contains either wasm or js

This likely will need a new test setup to serve DevTools in release build. I'm not sure the best package for this, whether we should be using integration_test or webdriver.