Supply a package root when editor runs analysis server from source.

Since the correct package root depends on the user's build
configuration and OS, we don't bother trying to infer it.  The user
should supply it using the "-Dcom.google.dart.packageRoot" option to
editor.

This only takes effect if the argument "-Dcom.google.dart.svnRoot" has
been supplied to force editor to run the analysis server from source.

R=brianwilkerson@google.com, scheglov@google.com

Review URL: https://codereview.chromium.org//671013002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41248 260f80e4-7a28-3924-810f-c04153c831b5