[Dart2] Fix bugs with maps could not be deserialized

Created by: ircecho

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, 3.3.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @ircecho (2017/07) @swipesight (2018/09)

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

Fixes the following problem when deserializing maps. Most likely due to stronger type checking in dart2.

Unhandled exception:
type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, Map<String, dynamic>>'
#0      new ArticleRepresentation.fromJson (file:///.../lib/model/article_representation.dart:85:59)
#1      ArticleRepresentation.listFromJson.<anonymous closure> (file:///.../lib/model/article_representation.dart:128:87)
...