We've been reluctant to make large-scale changes because they are likely to cause pain for people with unmerged PRs. But at this point I think there are relatively few open PRs that we are likely to merge, thanks largely to the heroic work of @Marcono1234. So this would be as good a time as any to consider at least:

• Port tests from JUnit 3 to JUnit 4. (Google's internal systems don't support JUnit 5 so we can't use that.)
• Use Truth for clearer assertions in tests.
• Apply various automated fixes suggested by Error Prone and the like.
• Reformat all source files with google-java-format. Possibly also install a check that ensures that future PRs are also correctly formatted.