I created this quick test to see if my intended method of expanding json_object would impact run time too much.

test_expanded_json_object.dart

import 'dart:mirrors';
import 'package:json_object/json_object.dart';
import 'package:unittest/unittest.dart';
import 'package:unittest/vm_config.dart';

class ControlClass{
  String A;
  int B;
  double C;
}

class TestClass{
  String A;
  int B;
  
  Map<String, Object> _backupData = {};
  
  TestClass.buildFromMap(Map map){
    _buildFromMap(map);
  }
  
  _buildFromMap(map){
    InstanceMirror im = reflect(this);
    ClassMirror cm = im.type;
    Map objData = cm.instanceMembers;
    map.forEach((key, value){
      Symbol keySymbol = new Symbol(key + "="); //adding "=" finds setters
      if (objData.containsKey(keySymbol)){
        im.setField(new Symbol(key), value);
      }
      else{
        _backupData[key] = value;
      }
    });
  }
  
  noSuchMethod(Invocation invo){
    if (invo.isAccessor && invo.isGetter){
      String memberName = MirrorSystem.getName(invo.memberName);
      return _backupData[memberName];
    } else {
      super.noSuchMethod(invo);
    }
  }
}

main(){
  useVMConfiguration();
  TestClass testObject;
  JsonObject jsonObject;
  ControlClass controlObject;
  var timer = new Stopwatch();
  var fieldMap = {"A":"Words","B":14,"C":4.59};
  
  setUp((){
    timer.start();
    testObject = new TestClass.buildFromMap(fieldMap);
    print("TestClass Created in ${timer.elapsedMilliseconds}ms");
    timer.reset();
    controlObject = new ControlClass()
      ..A = "Words"
      ..B = 14
      ..C = 4.59;
    print("ControlClass Created in ${timer.elapsedMilliseconds}ms");
    timer.reset();
    jsonObject = new JsonObject.fromMap(fieldMap);
    print("JsonObject Created in ${timer.elapsedMilliseconds}ms");
    timer.stop();
  });
  
  test('fieldAccess',(){
    timer.start();
    expect(testObject.A, equals("Words"));
    expect(testObject.B, equals(14));
    expect(testObject.C, equals(4.59));
    print("testObject accessed in ${timer.elapsedMilliseconds}ms");

    timer.reset();
    expect(controlObject.A, equals("Words"));
    expect(controlObject.B, equals(14));
    expect(controlObject.C, equals(4.59));
    print("controlObject accessed in ${timer.elapsedMilliseconds}ms");

    timer.reset();
    expect(jsonObject.A, equals("Words"));
    expect(jsonObject.B, equals(14));
    expect(jsonObject.C, equals(4.59));
    print("jsonObject accessed in ${timer.elapsedMilliseconds}ms");
    timer.stop();
  });
}

Creation times

TestClass Created in 28ms
ControlClass Created in 0ms
JsonObject Created in 5ms

Access times

testObject accessed in 23ms
controlObject accessed in 0ms
jsonObject accessed in 1ms