Provide flatten for sequence of iterables.

#KT-9977

libraries/stdlib/test/collections/SequenceTest.kt

@@ -319,8 +319,19 @@ public class SequenceTest {
     }
 
     @test fun flatten() {
-        val data = sequenceOf(1, 2).map { sequenceOf(0..it) }
-        assertEquals(listOf(0, 1, 0, 1, 2), data.flatten().toList())
+        val expected = listOf(0, 1, 0, 1, 2)
+
+        val seq = sequenceOf((0..1).asSequence(), (0..2).asSequence()).flatten()
+        assertEquals(expected, seq.toList())
+
+        val seqMappedSeq = sequenceOf(1, 2).map { (0..it).asSequence() }.flatten()
+        assertEquals(expected, seqMappedSeq.toList())
+
+        val seqOfIterable = sequenceOf(0..1, 0..2).flatten()
+        assertEquals(expected, seqOfIterable.toList())
+
+        val seqMappedIterable = sequenceOf(1, 2).map { 0..it }.flatten()
+        assertEquals(expected, seqMappedIterable.toList())
     }
 
     @test fun distinct() {