Dart-lang template bugs fixed with "baseName"

Created by: kuronekomichael

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

[Only Dart-lang]

• If the definition includes format: date, it will not be treated as a date type and will call an incorrect method DateTime.fromJson() which does not exist
• If the property name is underscore_case, the name will always be changed to lowerCamelcase and will be accessed with an incorrect property name

Example

swagger: '2.0'
info:
  version: 1.0.0
  title: Simplest OpenAPI Sample
paths:
  /me/status:
    get:
      operationId: getMyStatus
      consumes:
        - application/json
      produces:
        - application/json
      responses:
        '200':
          description: successful operation
          schema:
            $ref: '#/definitions/Status'
        '405':
          description: Invalid input
definitions:
  Status:
    type: object
    properties:
      created_at:
        type: string
        format: date-time
      birthDate:
        type: string
        format: date
      avatar_url:
        type: string

When generated from the above yaml file, the difference in output is as follows.

diff -r -u master/lib/model/status.dart fixed/lib/model/status.dart
@@ -16,15 +16,15 @@

   Status.fromJson(Map<String, dynamic> json) {
     if (json == null) return;
-    createdAt = json['createdAt'] == null ? null : DateTime.parse(json['createdAt']);
-    birthDate = new DateTime.fromJson(json['birthDate']);
-    avatarUrl = json['avatarUrl'];
+    createdAt = json['created_at'] == null ? null : new DateTime.fromMillisecondsSinceEpoch(json['created_at'].toInt() * 1000);
+    birthDate = json['birthDate'] == null ? null : DateTime.parse(json['birthDate']);
+    avatarUrl = json['avatar_url'];
   }

   Map<String, dynamic> toJson() {
     return {
       'createdAt': createdAt == null ? '' : createdAt.toUtc().toIso8601String(),
-      'birthDate': birthDate,
+      'birthDate': birthDate == null ? '' : birthDate.toUtc().toIso8601String(),
       'avatarUrl': avatarUrl
     };
   }

❌ json['createdAt'] is invalid property name. ✅ json['created_at'] is correct

❌ new DateTime.fromJson(json['birthDate']); throws an error. DateTime don't have fromJson method. ✅ DateTime.parse(json['birthDate']); is correct.

@ircecho Please help me!