Introduce new overloads of flatMap and flatMapTo

- Sequence<T>.flatMap((T) -> Iterable<R>)
- Iterable<T>.flatMap((T) -> Sequence<R>)
- Array<T>.flatMap((T) -> Sequence<R>)
- Map.flatMap((Entry) -> Sequence<R>)

KT-34506

libraries/stdlib/test/collections/MapTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -216,6 +216,21 @@ class MapTest {
         assertEquals(mapOf(8 to "Mells"), m3)
     }
 
+    @Test fun flatMap() {
+        fun <T> list(entry: Map.Entry<T, T>): List<T> = listOf(entry.key, entry.value)
+        fun <T> seq(entry: Map.Entry<T, T>): Sequence<T> = sequenceOf(entry.key, entry.value)
+        val m = mapOf("x" to 1, "y" to 0)
+        val result1 = m.flatMap { list(it) }
+        val result2 = m.flatMap { seq(it) }
+        val result3 = m.flatMap(::list)
+        val result4 = m.flatMap(::seq)
+        val expected = listOf("x", 1, "y", 0)
+        assertEquals(expected, result1)
+        assertEquals(expected, result2)
+        assertEquals(expected, result3)
+        assertEquals(expected, result4)
+    }
+
     @Test fun createFrom() {
         val pairs = arrayOf("a" to 1, "b" to 2)
         val expected = mapOf(*pairs)