Make TimeZone.UTC return FixedOffsetTimeZone (statically)

Author: ilya-g

core/common/src/TimeZone.kt

@@ -31,7 +31,7 @@ public expect open class TimeZone {
         /**
          * Returns the time zone with the fixed UTC+0 offset.
          */
-        public val UTC: TimeZone
+        public val UTC: FixedOffsetTimeZone
 
         /**
          * Returns the time zone identified by the provided [zoneId].

core/common/test/TimeZoneTest.kt

@@ -14,8 +14,10 @@ class TimeZoneTest {
 
     @Test
     fun utc() {
-        println(TimeZone.UTC)
-        assertEquals("Z", TimeZone.UTC.id)
+        val utc: FixedOffsetTimeZone = TimeZone.UTC
+        println(utc)
+        assertEquals("Z", utc.id)
+        assertEquals(0, utc.utcOffset.totalSeconds)
     }
 
     @Test

core/js/src/TimeZone.kt

@@ -27,7 +27,7 @@ public actual open class TimeZone internal constructor(internal val zoneId: Zone
 
     public actual companion object {
         public actual fun currentSystemDefault(): TimeZone = ZoneId.systemDefault().let(::TimeZone)
-        public actual val UTC: TimeZone = jtZoneOffset.UTC.let(::TimeZone)
+        public actual val UTC: FixedOffsetTimeZone = UtcOffset(jtZoneOffset.UTC).asTimeZone()
 
         public actual fun of(zoneId: String): TimeZone = try {
             val zone = ZoneId.of(zoneId)

core/jvm/src/TimeZoneJvm.kt

@@ -32,7 +32,7 @@ public actual open class TimeZone internal constructor(internal val zoneId: Zone
 
     public actual companion object {
         public actual fun currentSystemDefault(): TimeZone = ZoneId.systemDefault().let(::TimeZone)
-        public actual val UTC: TimeZone = jtZoneOffset.UTC.let(::TimeZone)
+        public actual val UTC: FixedOffsetTimeZone = UtcOffset(jtZoneOffset.UTC).asTimeZone()
 
         public actual fun of(zoneId: String): TimeZone = try {
             val zone = ZoneId.of(zoneId)

core/native/src/TimeZone.kt

@@ -18,7 +18,7 @@ public actual open class TimeZone internal constructor(internal val value: TimeZ
 
         public actual fun currentSystemDefault(): TimeZone = PlatformTimeZoneImpl.currentSystemDefault().let(::TimeZone)
 
-        public actual val UTC: TimeZone = UtcOffset.ZERO.asTimeZone()
+        public actual val UTC: FixedOffsetTimeZone = UtcOffset.ZERO.asTimeZone()
 
         // org.threeten.bp.ZoneId#of(java.lang.String)
         public actual fun of(zoneId: String): TimeZone {