Diagnostics can be easier to read if you see them in the order in which
they were generated. By selecting the compilation and indexes, they get
sorted by the testsuite driver.
d.getCompilationInfo(c, f, i) would be a bit more natural as
d = c.getDiagnostic(f, i), but currently we don't import Diagnostic into
the default ('import java') namespace, and I don't think it's worth
changing that for this.