Fix type error in test.dart script.

BUG=
R=eernst@google.com

Review URL: https://codereview.chromium.org/2264003002 .

tools/test.dart

@@ -41,7 +41,8 @@ Future _deleteTemporaryDartDirectories() {
   var environment = Platform.environment;
   if (environment['DART_TESTING_DELETE_TEMPORARY_DIRECTORIES'] == '1') {
     LeftOverTempDirPrinter.getLeftOverTemporaryDirectories().listen(
-        (Directory tempDirectory) {
+        (FileSystemEntity tempEntity) {
+          Directory tempDirectory = tempEntity as Directory;
           try {
             tempDirectory.deleteSync(recursive: true);
           } catch (error) {

tools/testing/dart/test_progress.dart

@@ -488,7 +488,7 @@ class LeftOverTempDirPrinter extends EventListener {
     }
   }
 
-  static Stream<Directory> getLeftOverTemporaryDirectories() {
+  static Stream<FileSystemEntity> getLeftOverTemporaryDirectories() {
     var regExp = _getTemporaryDirectoryRegexp();
     return Directory.systemTemp.list().where((FileSystemEntity fse) {
       if (fse is Directory) {