[firebase_performance] Testing of firebase performance rewrite PRs

[bparrishMines]

Description

Reformat firebase_performance plugin to enable the use of meaningful integration testing.

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[collinjackson]

This change LGTM. Minor nit though, I think that assigning test data to variables in unit tests is discouraged and that we're encouraging less code re-use in favor of making the tests more declarative. For example, instead of assigning value to 'apple' you could say 'apple' everywhere.

@amirh may have more opinions about this. I realize this guideline is a bit unintuitive and I don't feel strongly about it, probably we should write it down either way.

[bparrishMines]

I was thinking about this as well. I typically always use literals, but I wanted to make this PR a little shorter and be focused on the refactor while the next one will be focused on tests.