This CL does several things:
* Splits lib/path.dart into multiple libraries.
* Renames "Builder" to "Context".
* Makes the Context API match the top-level functions ("root" was
renamed to "current" and "resolve" was renamed to "absolute").
* The top-level "absolute" function now takes multiple parts, to match
[Context.absolute].
R=rnystrom@google.com
BUG=14981
Review URL: https://codereview.chromium.org//62753005
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@30423 260f80e4-7a28-3924-810f-c04153c831b5
nweiz@google.com