Call chain into sequence: add 'flatten' to conversion targets #KT-26969 Fixed

idea/testData/inspectionsLocal/collections/convertCallChainIntoSequence/flatten.kt

@@ -0,0 +1,4 @@
+// WITH_RUNTIME
+fun test() {
+    listOf(listOf(1), listOf(2)).<caret>flatten().filter { it > 1 }
+}

idea/testData/inspectionsLocal/collections/convertCallChainIntoSequence/flatten.kt.after

@@ -0,0 +1,4 @@
+// WITH_RUNTIME
+fun test() {
+    listOf(listOf(1), listOf(2)).asSequence().flatten().filter { it > 1 }.toList()
+}

idea/testData/inspectionsLocal/collections/convertCallChainIntoSequence/flatten2.kt

@@ -0,0 +1,4 @@
+// WITH_RUNTIME
+fun test() {
+    listOf(listOf(1), listOf(2, 3)).<caret>filter { it.size > 1 }.flatten()
+}

idea/testData/inspectionsLocal/collections/convertCallChainIntoSequence/flatten2.kt.after

@@ -0,0 +1,4 @@
+// WITH_RUNTIME
+fun test() {
+    listOf(listOf(1), listOf(2, 3)).asSequence().filter { it.size > 1 }.flatten().toList()
+}