Use HttpRequest.uri.queryParameters.

HttpRequest.queryParameters is deprecated. (See
https://groups.google.com/a/dartlang.org/d/msg/misc/Y2UrfSEW6uA/-ZKCV-vLYUcJ)

This is a breaking change, so people not using the newest version of Dart would probably want to hold off on upgrading. Do you want me to iterate the Start version number to indicate this is a breaking change?