Make ClockMark an abstract class instead of an interface

libraries/stdlib/js/src/kotlin/time/MonoClock.kt

@@ -30,7 +30,7 @@ internal external interface Process {
 
 internal class HrTimeClock(val process: Process) : Clock {
 
-    override fun mark(): ClockMark = object : ClockMark {
+    override fun mark(): ClockMark = object : ClockMark() {
         val startedAt = process.hrtime()
         override fun elapsed(): Duration =
             process.hrtime(startedAt).let { (seconds, nanos) -> seconds.seconds + nanos.nanoseconds }