Clean and create output directory when generating docs.

frog/file_system_dom.dart

Index: frog/file_system_dom.dart
diff --git a/frog/file_system_dom.dart b/frog/file_system_dom.dart
index c245edcb062e3310d041eac99ec1b336eebeba26..35f0898b771e0ccdbf364d6072e8e4229b522024 100644
--- a/frog/file_system_dom.dart
+++ b/frog/file_system_dom.dart
@@ -68,4 +68,14 @@ class DomFileSystem implements FileSystem {
 
   // Note: this is not a perf nightmare only because of caching.
   bool fileExists(String filename) => readAll(filename) != null;
+
+  void createDirectory(String path, [bool recursive = false]) {
+    // TODO(rnystrom): Implement.
+    throw 'createDirectory() is not implemented by DomFileSystem yet.';
+  }
+
+  void removeDirectory(String path, [bool recursive = false]) {
+    // TODO(rnystrom): Implement.
+    throw 'removeDirectory() is not implemented by DomFileSystem yet.';
+  }
 }