APIPort flags API as missing when override is missing

[ericstj]

For example: DirectoryInfo.FullName.

On desktop this is:

public class FileSystemInfo {
 public virtual string FullName { get; }
}
public class DirectoryInfo: FileSystemInfo {
 public override string FullName { get; }
}

On .NETCore this is:

public class FileSystemInfo {
 public virtual string FullName { get; }
}
public class DirectoryInfo: FileSystemInfo {
}

APIPort reports DirectoryInfo.FullName as missing but it is not.