Improve the "pub lish" warnings about dependency constraints.

This adds a warning for a constraint that only allows a single version
and warnings for constraints with no upper bound or no lower bound. It
also improves the upper bound on suggested pre-1.0 constraints to
follow the current Dart team practice and fixes a bug in
[VersionRange.isAny].

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//173113004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@32867 260f80e4-7a28-3924-810f-c04153c831b5