Dart API docs: add support for generation and display

[chalin]

Supersedes #1593.

Task breakdown

Package dependencies:

• [Issue moved to [Dart] API doc generation - update harp version and eliminate patch #1899] Update to new harp package which depends on latest terraform release. This is needed so that dots (.) can be supported in site URIs; something that is essential for Dart given that library names contain dots. See Update to Terraform v1.0.0 sintaxi/harp#526 (/cc @sintaxi).
  Blocked awaiting the new release.

Gulp tasks / build scripts:

• New tasks to run pub and dartdoc over (existing) angular-dart repo, peer to ng.io repo.
• build-api-docs should also do Dart API docs.
• Shredder should run over Dart sources and generate example fragments.
• Create script to patch local terraform package, while we await new harp release, as a means of unblocking.

Generator package:

• New tools/dart-api-builder; stand alone tool re-using part of the basis of a Dart API dgeni package.

Harp/Jade site infrastructure:

• New public/docs/_layout-dart-api.jade.
• Updates to other Harp/Jade template files.

API List directive:

• Create new or update existing <api-list> directive:
  • Display only filter categories that are relevant to Dart.
  • Make library header a link to the library "index" page.

Site data and Jade file generation:

• Main API page
  • Generate api-list.json.
  • Add warning that API is preliminary and that code might still appear as TS.
• Generate data (_data.json) and Jade files for
  • For each library (i.e., top-level folder).
    • _data.json
    • Library "index" Jade page.
    • Each member Jade page.
  • For library subfolders (e.g., containing class member details).
    • _data.json
    • Folder "index" page (it actually has the same name as the container -- e.g., class name).
    • Each member page.
• API Jade file page contents:
  • Attributes like name and type
  • Breadcrumbs
  • Page header section tabs.
  • Core page content excerpt from dartdoc-generated page
    • Exclude the "left" nav menu (full ng2 package content list) from the core page content.
  • {@example ...} tags are replaced by their designated example code fragments.
• Filter generated API pages (configurable via regExp in build file): exclude entries from
  • Non-public, developer/internal libraries.
  • Top-level catch all angular2 library -- otherwise every entry appears twice.
• Support developer & production modes for data (JSON) file generation
  • Developer mode (pretty-printed).
  • [NOT implemented] Production mode (compact).

cc: @naomiblack @kwalrath @ferhatb @keertip