True, but [this code](https://gist.github.com/9125442) references `_output` it as a StringBuffer.  I haven't looked deep into `_outputProvider` to see what `compiler_unsupported` all does with it during compilation.  The only time that we initialize `_output` with a `String` is once we have received `CompilerResults` from the service.  The alternative to this is having two separate, but mostly identical classes on either end (host and isolate).  My vote is to keep `CompilerResult` as common between the two ends and, on the host end, either `import "compiler.dart" show CompilerResult;` or put it in a separate (common) dart file.