Clean and create output directory when generating docs.

frog/file_system_vm.dart

Index: frog/file_system_vm.dart
diff --git a/frog/file_system_vm.dart b/frog/file_system_vm.dart
index 29c1c59b51cdb6d3cf5f680bfca2e4f5d0b28b01..306fcd3214f137d12e7a0eb7f350ad45f09ebb47 100644
--- a/frog/file_system_vm.dart
+++ b/frog/file_system_vm.dart
@@ -27,4 +27,14 @@ class VMFileSystem implements FileSystem {
   bool fileExists(String filename) {
     return new File(filename).existsSync();
   }
+
+  void createDirectory(String path, [bool recursive = false]) {
+    // TODO(rnystrom): Implement.
+    throw 'createDirectory() is not implemented by VMFileSystem yet.';
+  }
+
+  void removeDirectory(String path, [bool recursive = false]) {
+    // TODO(rnystrom): Implement.
+    throw 'removeDirectory() is not implemented by VMFileSystem yet.';
+  }
 }