[dart] "type: object" with no properties generates invalid code

Description

Generating a swagger api that has definitions of type=object and no properties generates Object.mapFromJson(json['parameterBag']). This code is not valid and it should(I think) generate json['parameterBag'].

The api I am using is returning a map with key value pairs that are dynamic. That is why there are no properties defined in the swagger file.

Swagger-codegen version

2.4.1

Swagger declaration file content or url

{
    "host": "localhost",
    "basePath": "/",
    "produces": [
        "application/json"
    ],
    "consumes": [
        "application/json"
    ],
    "schemes": [
        "http"
    ],
    "swagger": "2.0",
    "definitions": {
        "SendRequest": {
            "properties": {
                "parameterBag": {
                    "additionalProperties": {
                        "type": "object"
                    },
                    "type": "object"
                }
            },
            "type": "object"
        },
        "SendResponse": {
            "$schema": "http://json-schema.org/draft-04/schema#",
            "properties": {
                "msg": {
                    "type": "string"
                }
            },
            "type": "object"
        }
    },
    "paths": {
        "/send": {
            "post": {
                "description": "",
                "operationId": "send",
                "parameters": [
                    {
                        "description": "",
                        "in": "body",
                        "name": "body",
                        "required": true,
                        "schema": {
                            "$ref": "#/definitions/SendRequest"
                        }
                    }
                ],
                "responses": {
                    "200": {
                        "description": "Success",
                        "schema": {
                            "$ref": "#/definitions/SendResponse"
                        }
                    },
                    "400": {
                        "description": "Validation incorrect"
                    },
                    "404": {
                        "description": "Not found"
                    }
                }
            }
        }
    }
}

Command line used for generation

java -jar swagger-codegen-cli.jar generate -i dart.json -l dart -o generated_code/ -D browserClient=false

Steps to reproduce

• Use type object with no properties
• Compile the code

Suggest a fix/enhancement

If the type has no properties do not render the Object.mapFromJson

Run the supplied swagger-codegen-cli command with the declaration in the issue and inspect the Dart generator templates that produce parameterBag deserialization. Verify the generated client compiles and that an empty object schema no longer emits the invalid Object.mapFromJson call.