docs(contributing): add code of conduct
revert: refactor(mocks): simplify the `inject` implementation

This reverts commit 64d58a5b5292046adf8b28928950858ab3895fcc.

For some weird reason this is causing regressions at Google.
I'm not sure why and I'm running out of time to investigate, so I'm taking
a safe route here and reverting the commit since it's just a refactoring.