Move all mutable state from FileTreeWalk to its iterator implementation, allowing this sequence to be iterated several times.

Introduce SingleFileState for single file walks.
Do not check isDirectory in DirectoryState constructor unless assertions are enabled.

libraries/stdlib/test/io/FileTreeWalks.kt

@@ -49,6 +49,23 @@ class FileTreeWalkTest {
         }
     }
 
+    @Test fun singleFile() {
+        val testFile = createTempFile()
+        val nonExistantFile = testFile.resolve("foo")
+        try {
+            for (walk in listOf(File::walkTopDown, File::walkBottomUp)) {
+                assertEquals(testFile, walk(testFile).single(), "${walk.name}")
+                assertTrue(walk(testFile).treeFilter { false }.none(), "${walk.name}")
+                assertEquals(testFile, testFile.walk().onEnter { false }.single(), "${walk.name} - enter should not be called for single file")
+
+                assertTrue(walk(nonExistantFile).none(), "${walk.name} - enter should not be called for single file")
+            }
+        }
+        finally {
+            testFile.delete()
+        }
+    }
+
     @Test fun withEnterLeave() {
         val basedir = createTestFiles()
         try {