[Dart2] Fix bugs with maps could not be deserialized

[ircecho]

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  @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)
...

[ghost]

I found some info about this issue in https://www.dartlang.org/guides/language/sound-problems
Seems like can use cast to fix.

In cases where you are working with a collection that you don’t create, such as from JSON or an external data source, you can use the cast() method provided by List and other collection classes.

Here’s an example of the preferred solution: tightening the object’s type.

Map<String, dynamic> json = getFromExternalSource();
var names = json['names'] as List;
assumeStrings(names.cast<String>());

[ircecho]

Your suggestion may be another solution. However, then one presented in my Pull Request works perfectly and I do not see any necessity to use cast. The method listFromJson and uses the same typing scheme and has been for years.

[wing328]

I suggest we go with this fix and consider using cast later if needed.

If no more question/feedback, I'll merge this PR tomorrow.

[wing328]

@ircecho thanks again for the PR, which is included in the v3.3.0 minor release: https://twitter.com/oas_generator/status/1046941449609068544