[Dart][API] Support angular.io doc relative links and code-regions

[chalin]

Here is an API doc excerpt illustrating this issue's use case

/* ...
 * though it is better practice to bind to a component field or method. Try the
 * [live example][ex] from the [Template Syntax][guide] chapter. Here are the
 * relevant excerpts from the example's template and component class:
 *
 * {@example docs/template-syntax/lib/app_component.html region=NgStyle}
...
 * [guide]: docs/guide/template-syntax.html#!#ngStyle
 * [ex]: examples/template-syntax/#ngStyle
 */

• @example URL starting with docs refers to NgStyle fragment of the named template-syntax example. By default @example fetches code from the API examples folder.
• Links with URLs starting with either docs or examples refer to (respectively):
  • angular.io/docs/dart/latest chapters
  • http://angular-examples.github.io/ "live" apps

cc @kwalrath

[chalin]

Closed via #2050.