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.
