Make sure package names are URL encoded.

If you happen to have a weird package name like "/* blah *?" you
would get a nasty HTTP error when it tried to jam that in a URL.
Instead, this will give you a friendlier 'package "/* blah */" not found error."
BUG=http://code.google.com/p/dart/issues/detail?id=7466

Review URL: https://codereview.chromium.org//12328021

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@18849 260f80e4-7a28-3924-810f-c04153c831b5