Add tests for flat-mapping operations

idea/testData/debugger/sequence/exec/sequence/outs/flatMap.out

@@ -0,0 +1,32 @@
+LineBreakpoint created at FlatMap.kt:5
+Run Java
+Connected to the target VM
+FlatMap.kt:5
+listOf(1, 0, 2).asSequence()
+.flatMap({ (0 until it).asSequence() })
+.count()
+flatMap
+    before: 1,3,4
+    after: 2,5,6
+count
+    before: 2,5,6
+    after: nothing
+mappings for flatMap
+  direct:
+    1 -> 2
+    3 -> nothing
+    4 -> 5,6
+  reverse:
+    1 <- 2
+    4 <- 5
+    4 <- 6
+mappings for count
+  direct:
+    2 -> nothing
+    5 -> nothing
+    6 -> nothing
+  reverse:
+    empty
+Disconnected from the target VM
+
+Process finished with exit code 0

idea/testData/debugger/sequence/exec/sequence/outs/flatten.out

@@ -0,0 +1,38 @@
+LineBreakpoint created at Flatten.kt:5
+Run Java
+Connected to the target VM
+Flatten.kt:5
+listOf(listOf(1,2,3), listOf(), listOf(4,5,6)).asSequence()
+.flatten()
+.toList()
+flatten
+    before: 1,5,6
+    after: 2,3,4,7,8,9
+toList
+    before: 2,3,4,7,8,9
+    after: nothing
+mappings for flatten
+  direct:
+    1 -> 2,3,4
+    5 -> nothing
+    6 -> 7,8,9
+  reverse:
+    1 <- 2
+    1 <- 3
+    1 <- 4
+    6 <- 7
+    6 <- 8
+    6 <- 9
+mappings for toList
+  direct:
+    2 -> nothing
+    3 -> nothing
+    4 -> nothing
+    7 -> nothing
+    8 -> nothing
+    9 -> nothing
+  reverse:
+    empty
+Disconnected from the target VM
+
+Process finished with exit code 0

idea/testData/debugger/sequence/exec/sequence/src/flatMap/FlatMap.kt

@@ -0,0 +1,6 @@
+package flatMap
+
+fun main(args: Array<String>) {
+  // Breakpoint!
+  listOf(1, 0, 2).asSequence().flatMap { (0 until it).asSequence() }.count()
+}

idea/testData/debugger/sequence/exec/sequence/src/flatMap/Flatten.kt

@@ -0,0 +1,6 @@
+package flatMap
+
+fun main(args: Array<String>) {
+  // Breakpoint!
+  listOf(listOf(1,2,3), listOf(), listOf(4,5,6)).asSequence().flatten().toList()
+}