Add http_parser to dependencies; alphabetize

[srawlins]

http_parser was actively being used by redstone, but the dependency must have been met transitively:

$ git grep http_parser
lib/src/request_mock.dart:import 'package:http_parser/src/media_type.dart';

[Pacane]

Is alphabetizing the pubspec dependencies something in the Dart guideline?

[srawlins]

Mmm it might be... I just did it to make dependencies easier to find.

[Pacane]

all right, well thanks! You might want to check though. I think there are places where the /src path was imported. maybe could you also replace those references with the package notation?