Introduce reversed extension for Comparator.

libraries/stdlib/src/kotlin/util/Ordering.kt

@@ -315,4 +315,15 @@ public fun <T: Comparable<T>> nullsLast(): Comparator<T?> {
             return a.compareTo(b)
         }
     }
-}
+}
+
+/** Returns a comparator that imposes the reverse ordering of this comparator. */
+public fun <T> Comparator<T>.reversed(): Comparator<T> = when (this) {
+    is ReversedComparator -> this.comparator
+    else -> ReversedComparator(this)
+}
+
+
+private class ReversedComparator<T>(public val comparator: Comparator<T>): Comparator<T> {
+    override fun compare(a: T, b: T): Int = comparator.compare(b, a)
+}

libraries/stdlib/test/collections/CollectionTest.kt

@@ -589,7 +589,12 @@ class CollectionTest {
     }
 
     test fun sortedWith() {
-        expect(listOf("BAD", "dad", "cat")) { listOf("cat", "dad", "BAD").sortedWith(compareBy { it.toUpperCase().reverse() }) }
+        val comparator = compareBy<String> { it.toUpperCase().reverse() }
+        val data = listOf("cat", "dad", "BAD")
+
+        expect(listOf("BAD", "dad", "cat")) { data.sortedWith(comparator) }
+        expect(listOf("cat", "dad", "BAD")) { data.sortedWith(comparator.reversed()) }
+        expect(listOf("BAD", "dad", "cat")) { data.sortedWith(comparator.reversed().reversed()) }
     }
 
     test fun decomposeFirst() {