[BUG] [dart][dart-dio] Generator does not work with property named "source" or definition named "URL"

Bug Report Checklist

• [ x] Have you provided a full/minimal spec to reproduce the issue?
• [ x] Have you validated the input using an OpenAPI validator (example)?
• [ x] What's the version of OpenAPI Generator used?
• [x ] Have you search for related issues/PRs?
• [x ] What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix ([example]

Description

When trying to generate dart-dio source from a json openapi definition that includes a property called "source" or a definition called "URL", it does not compile

openapi-generator version

MacOS Catalina, openapi-generator 4.2.2

OpenAPI declaration file content or url

...
"definitions": {
  "URL": {
     "type": "object",
     "properties": {
        "myproperty": {
        "type": "string"
       }
    }
  },
  "Document":{
    "type": "object",
    "properties": {
      "source": {
       "type": "string"
    }
  }
 }
}
...

Command line used for generation

openapi-generator generate -g dart-dio -i openapi.json -o openapi/

Steps to reproduce

1. create openapi.json with a definition called "URL" and / or a field called "source"
2. generate dart-dio source
3. try building the source with mentioned command-line below

Error in BuiltValueGenerator for /openapi/lib/model/url.dart.
Please make the following changes to use built_value serialization:

1. Declare URL.serializer as: static Serializer<URL> get serializer => _$uRLSerializer; got static Serializer<URL> get serializer => _$URLSerializer;

Error in BuiltValueGenerator for abstract class Document implements Built<Document, dynamic>.
Please make the following changes to use BuiltValue:

1. Make field _source public; remove the underscore.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/4865

Suggest a fix

renaming the variables in the dart source like mentioned in the error messages works:
_source -> source
_$URLSerializer -> _$uRLSerializer (for the instance variable, not the class)

Start at the dart-dio generator entry point and reproduce the issue with the provided openapi.json containing a source property and URL definition. Run flutter pub run build_runner build --delete-conflicting-outputs; done means the generated Dart source builds without the reported serializer or field errors.