dart: Escape HTML in todo rendering

I took the implementation from the [experimental branch](https://code.google.com/p/dart/source/browse/experimental/lib_v2/dart/pkg/htmlescape/lib/htmlescape.dart). I guess at some point in the future we can use the library function.

@MathieuLorber Could you take a quick look if this is okay?

(ref #397)