Fix Directory.create(recursive: true) for non-existing paths.

tests/standalone/io/regress_7679_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:io';
+
+main() {
+  Directory temp = new Directory('').createTempSync();
+  File script = new File('${temp.path}/script.dart');
+  script.writeAsStringSync("""
+import 'dart:io';
+
+main() {
+  Directory d = new Directory('a');
+  d.create(recursive: true).then((_) {
+    d.exists().then((result) {
+      Expect.isTrue(result);
+      d = new Directory('b/c/d');
+      d.create(recursive: true).then((_) {
+        d.exists().then((result) {
+          Expect.isTrue(result);
+        });
+      });
+    });
+  });
+}
+""");
+  ProcessOptions options = new ProcessOptions();
+  options.workingDirectory = temp.path;
+  String executable = new File(new Options().executable).fullPathSync();
+  Process.run(executable, ['script.dart'], options).then((result) {
+    temp.deleteSync(recursive: true);
+    Expect.equals(0, result.exitCode);
+  });
+}