For example: https://ci.chromium.org/p/flutter/builders/prod/Mac%20beta%20iOS%20Engine%20Release/85

This happened because the version of Xcode across Flutter's LUCI builds was incremented from Xcode 11 -> Xcode 12, and the newer version has different clang settings, causing a compilation error in the skia source. This was fixed upstream by: https://skia-review.googlesource.com/c/skia/+/418636

I worked around this by re-running affected builds, overriding the Xcode version with Xcode 11.