Previously, builds were failing, even a re-build of previously passing master. The output from bin/test included the following warning which looked like a possible root cause:
warning You are using Node "4.2.6" which is not supported and may encounter bugs or unexpected behavior. Yarn supports the following semver range: "^4.8.0 || ^5.7.0 || ^6.2.2 || ^8.0.0"
Sure enough, specifying a recent node version in circle.yml gets the build back to green.
purinkle
commented
5 years ago 
Previously, builds were failing, even a re-build of previously passing
master. The output frombin/testincluded the following warning which looked like a possible root cause:Sure enough, specifying a recent node version in
circle.ymlgets the build back to green.