Change dartdocgen --serve to use pub get rather than pub upgrade

BUG=
R=sethladd@google.com

Review URL: https://codereview.chromium.org//806953002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42376 260f80e4-7a28-3924-810f-c04153c831b5
Issue 21883. Add checks if type parameters can be used.

R=brianwilkerson@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=21883

Review URL: https://codereview.chromium.org//806933002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42377 260f80e4-7a28-3924-810f-c04153c831b5