code: public Period plusYears(long yearsToAdd) {
          if (yearsToAdd == 0) {
              return this;
          }
          return create(Jdk8Methods.safeToInt(Jdk8Methods.safeAdd(years, yearsToAdd)), months, days);
      }
docs: Returns a copy of this period with the specified years added.
  <p>
  This adds the amount to the years unit in a copy of this period.
  The months and days units are unaffected.
  For example, "1 year, 6 months and 3 days" plus 2 years returns "3 years, 6 months and 3 days".
  <p>
  This instance is immutable and unaffected by this method call.
  
  @param yearsToAdd  the years to add, positive or negative
  @return a {@code Period} based on this period with the specified years added, not null
  @throws ArithmeticException if numeric overflow occurs