Gist of a simple dart http server that can handle JSON with an inconsistent schema.

dserve.dart

import 'dart:io';
import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart';

void main(List<String> args) async {

	String url, port;

	try { url = args[0]; } catch(e) { print("Need a url"); exit(-1); }
	try { port = args[1]; } catch(e) { print(e); print("Need a port"); exit(-2); }
	
	String requrl = url+":"+port;
	print(requrl);

	// binds socket from OS, stores in server obj
	var server = await HttpServer.bind(InternetAddress.loopbackIPv4, int.parse(port));
	print("Listening on localhost:${server.port}");

	// asynchronously wait for requests to the server
	await for (HttpRequest request in server) {
	    final contenttype = request.headers.contentType; //contentType header of req
	    final response = request.response; // response body of the httpreq (has the json)
		print(contenttype);

		if (request.headers.contentType != null) print(request);
		
		if(request.headers.contentType.toString() == "application/json") {
			//decode request, string on each line
			//.join() the strings together
			final content = await utf8.decoder.bind(request).join();
			final responseJson = jsonDecode(content) as Map; // json -> Map<String,String>
			
			stdout.write("Got the following json: \n\n");
			print(responseJson);
			request.response.write("\n We got your JSON!\n"); // tell client
		} else {
			request.response.write("\n No JSON? What gives?\n");
		}
		print("Closing Response");
		await request.response.close();
	}

}

pubspec.yaml

name: jsontest
dependencies:
  json_serializable: ^3.1.0
  http: ^0.12.0+2

sample curls

curl -X POST localhost:2020 -H "Content-Type:application/json" -d '{"source":"xxx","source_pdf_filename":"demo_large_3.pdf","output_pdf_filename":"output.pdf","coordinates":["2808.5","111.7","2987.2","392.0"],"image_filename":"logo.png","stamp_text":"01.01.2000\nJoe Smith\nLicn# 31XI01508480","logo_text_size":"12"}' 

curl -X POST localhost:2020 -H "Content-Type:application/json" -d '{"hello":"world"}'