* chore(ng2/dart): update from beta.1x to beta.18
Update made to all `pubspec.yaml` files and to relevant prose.
Note that pipes was back a beta.15; the rest were at beta.17.
* chore(pipes/dart): update ngFor microsyntax and transform method param
Updates to pipe example source code:
- Pipe `transform()` method parameters
- `ngFor` microsyntax, from `#` to `let`
closes#1910
- Installs Dart SDK
- Installs ng2/dart sources from the pub package site.
- Re-enables doc build on Travis
- install Dartium browser for eventual testing.
- allow TMP and PKG to be set by developer before calling, e.g., install-dart-sdk.sh.
- removes `npm install -g gulp --no-optional` has been removed since it does not appear to be needed.
- Implements most of #1907.
