Add json parser reporting accurate source locations.

The json parser is a copy of "package:json/json.dart" where the parser has been modified to report accurate source locations (in absolute position from the beginning of the contents) to the [JsonListener] interface.

The parser will be used to implement "smarter" validators for json file with a specific format (e.g. manifest.json).
And there are still more in this file and others. Could you go over all of them and fit them into the 80-char limit?