At least the Dart generation, and probably all of the language generation, doesn't create the output file recursively. That leads to confusing flows like the following, which has bitten me several times:

1. Set up Pigeon for the first time in a plugin that was simple enough to only have a lib/plugin_name.dart file with all of the implementation.
2. Configure the Dart to output to lib/src/messages.g.dart.
3. Run Pigeon generation.
4. Get an error about lib/src/messages.g.dart not existing. (The actual problem is that lib/src/ doesn't exist, so it can't make the file).

We should do a create(recursive: true) on the target File before trying to write to it.