Wrong answers in some cases when adding date-based units to instants on Darwin and Windows

[dkhalanskyjb]

There is a rare scenario where the result may be off (usually by an hour) when the result of an addition to an Instant is in a time overlap and the source Instant has the lesser offset of the two possible in the overlap. Then, the overlap is resolved incorrectly.

val zone = TimeZone.of("Europe/Berlin")
val instant1 = LocalDateTime(2019, Month.OCTOBER, 27, 2, 59).toInstant(zone)
val instant2 = LocalDateTime(2019, Month.OCTOBER, 28, 2, 59).toInstant(zone)
val instant3 = instant1.minus(300, DateTimeUnit.DAY, zone)
val instant4 = instant1.minus(10, DateTimeUnit.MONTH, zone)
assertEquals(instant1.plus(DateTimeUnit.HOUR), instant2.plus(-1, DateTimeUnit.DAY, zone))
assertEquals(instant1.plus(DateTimeUnit.HOUR), instant3.plus(300, DateTimeUnit.DAY, zone))
assertEquals(instant1.plus(DateTimeUnit.HOUR), instant4.plus(10, DateTimeUnit.MONTH, zone))

All these assertions fail on Darwin and Windows but pass in the other implementations (JVM, Unix, JS).