closes#1818
Previously, the markdown of some chapters was converted to Jade markup
to support the conditional exclusion of TS prose parts in Dart
chapters. This commit reverts some of that back to clean markdown,
thanks to a few custom directives.
This fixes various issues such as:
- missing space in Dart version between the live demo and demo source
links.
- Live example link directing to Plunker for Dart.
The main mixins (`liveExampleLink`*) used to support the generation of
links now assume reasonable default values for their arguments, and in
most cases the defaults are used.
* Dart example links: use http vs https
As agree to with @kwalrath.