private static StringBuilder appendQuoted(StringBuilder sb, String s) {
    sb.append('"');
    for (int i = 0; i < s.length(); i++) {
      appendEscaped(sb, s.charAt(i));
    }
    return sb.append('"');
  }
Throws an exception if this annotation contains a value for a Class element that is not
actually a type. The assumption is that the value is the string {@code "<error>"} which javac
presents when a Class value is an undefined type.
value/src/main/java/com/google/auto/value/processor/AnnotationOutput.java