Add proper support for wasm in `package:web_benchmarks`.

[kenzieschmoll]

The test that covers wasm (or attempted to) was disabled in flutter/packages@0b86a4c.

Now that DevTools has been fully unblocked for running with wasm (flutter/devtools#7718 fixed the final TODO on flutter/devtools#6606), this is a great opportunity to run the existing DevTools performance benchmarks against dart2js and dart2wasm, and compare the results.

Providing benchmarking support through package:web_benchmarks will be useful for end users who want to try dart2wasm and verify its effectiveness at boosting performance.

CC @eyebrowsoffire @kevmoo

[github-actions]

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