Provide flatMapIndexed operation

- similar to flatMap, but transform function takes index and element

#KT-36894

libraries/stdlib/samples/test/samples/collections/collections.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * 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.
  */
 
 package samples.collections
@@ -549,6 +538,14 @@ class Collections {
             assertPrints(list.flatMap { it.toList() }, "[1, 2, 3, 4, 5]")
         }
 
+        @Sample
+        fun flatMapIndexed() {
+            val data: List<String> = listOf("Abcd", "efgh", "Klmn")
+            val selected: List<Boolean> = data.map { it.any { c -> c.isUpperCase() } }
+            val result = data.flatMapIndexed { index, s -> if (selected[index]) s.toList() else emptyList() }
+            assertPrints(result, "[A, b, c, d, K, l, m, n]")
+        }
+
         @Sample
         fun take() {
             val chars = ('a'..'z').toList()