static void deferIfMissingTypesIn(ExecutableElement method) {
    MISSING_TYPE_VISITOR.check(method.getReturnType());
    for (VariableElement param : method.getParameters()) {
      MISSING_TYPE_VISITOR.check(param.asType());
    }
  }
Check that the return type and parameter types of the given method are all defined, and arrange
to defer processing until the next round if not.

@throws MissingTypeException if the return type or a parameter type of the given method is
    undefined
value/src/main/java/com/google/auto/value/processor/MissingTypes.java