KT-3290 File.recurse NPE

#KT-3290 fixed

libraries/stdlib/src/kotlin/io/Files.kt

@@ -12,8 +12,9 @@ import java.net.URL
  */
 public fun File.recurse(block: (File) -> Unit): Unit {
     block(this)
-    if (this.isDirectory()) {
-        for (child in this.listFiles()!!) {
+    val children = this.listFiles()
+    if (children != null) {
+        for (child in children) {
             child.recurse(block)
         }
     }

libraries/stdlib/test/IoTest.kt

@@ -110,4 +110,29 @@ class IoTest(){
         assertEquals("", file1.relativePath(file1))
         assertEquals(file3.canonicalPath, file1.relativePath(file3))
     }
+
+    test fun listFiles() {
+        val dir = File.createTempFile("temp", System.nanoTime().toString())
+        dir.delete()
+        dir.mkdir()
+
+        File.createTempFile("temp", "1.kt", dir)
+        File.createTempFile("temp", "2.java", dir)
+
+        val subdir = File(dir, "subdir")
+        subdir.mkdir()
+
+        File(subdir, "3.txt").createNewFile()
+
+        var totalFiles = 0
+        dir.recurse { totalFiles++ }
+
+        assertEquals(5, totalFiles)
+
+        subdir.setReadable(false)
+
+        var totalFilesWithUnReadableDir = 0
+        dir.recurse { totalFilesWithUnReadableDir++ }
+        assertEquals(4, totalFilesWithUnReadableDir)
+    }
 }