code: func schemaDiff(before, after bigquery.Schema) string {
  	ret, err := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
  		A:        difflib.SplitLines(schemaString(before)),
  		B:        difflib.SplitLines(schemaString(after)),
  		FromFile: "Current",
  		ToFile:   "New",
  		Context:  3,
  		Eol:      "\n",
  	})
  	if err != nil {
  		// GetUnifiedDiffString returns an error only if it fails
  		// to write to a bytes.Buffer, which either cannot happen or we better
  		// panic.
  		panic(err)
  	}
  	return ret
  }
docs: // schemaDiff returns unified diff of two schemas.
  // Returns "" if there is no difference.