Better handling of query parameter names with `.` in them.

[lrhn]

See: http://dartbug.com/29520 or http://stackoverflow.com/questions/43729880 for context.

If a query parameter contains a ., the Symbol constructor will accept it (because #foo.bar is a valid symbol that can represent a library name), but it won't match any parameter name. It seems some support for this case is needed.