chore(amplify-codegen): dart api model .fromJson() refactor

[Equartey]

Description of changes

Flatens the nested data structure of serializedData found in the .fromJson() method on codegen models. This PR is to be paired with changes in amplify-flutter#4665

Issue #, if available

aws-amplify/amplify-flutter#816

Description of how you validated changes

Change were validated via:

1. amplify-codegen tests
2. amplify-flutter api category unit & integ tests
3. amplify-flutter datastore category unit & integ tests
4. Manual testing within both categories on ios & android devices

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• Breaking changes to existing customers are released behind a feature flag or major version update
• Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #593 (9b143c7) into main (c9a5c3b) will increase coverage by 0.00%.
The diff coverage is 85.71%.

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@@           Coverage Diff            @@
##             main     #593    +/-   ##
========================================
  Coverage   85.69%   85.69%            
========================================
  Files         148      153     +5     
  Lines        7380     7480   +100     
  Branches     1962     1976    +14     
========================================
+ Hits         6324     6410    +86     
- Misses        959      972    +13     
- Partials       97       98     +1     

Impacted Files	Coverage Δ
packages/graphql-docs-generator/src/cli.ts	0.00% <0.00%> (ø)
...s/graphql-docs-generator/src/generator/generate.ts	91.66% <ø> (ø)
...es/amplify-codegen/src/utils/readSchemaFromFile.js	33.33% <33.33%> (ø)
packages/graphql-docs-generator/src/index.ts	70.90% <68.00%> (-14.10%)	⬇️
.../graphql-docs-generator/src/generator/getFields.ts	79.54% <80.00%> (+1.76%)	⬆️
...ackages/amplify-codegen/src/commands/statements.js	85.91% <81.48%> (-1.59%)	⬇️
...phql-docs-generator/src/generator/utils/loading.ts	86.36% <89.47%> (-2.53%)	⬇️
...ql-docs-generator/src/generator/utils/templates.ts	91.66% <91.66%> (ø)
.../amplify-codegen/src/callbacks/postPushCallback.js	92.85% <100.00%> (ø)
...mplify-codegen/src/callbacks/prePushAddCallback.js	62.16% <100.00%> (ø)
... and 14 more

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[Jordan-Nelson]

Q: Can we be sure that json['${varName}'] is a List at this point?

The existing logic only casts to a List after checking that the type is List. The new logic casts to a List after checking that it is not null and not a Map. If json['${varName}'] were ever something other than Map, List, or null this would throw an exception.

The logic in the PR is:

• If map, handle map
• Otherwise, if not null, cast to List and handle list
• Otherwise return null

Would it make more sense to have:

• If map, handle map
• Otherwise, if List, handle list
• Otherwise, return null

[Equartey]

Good call out. I adjusted the logic to prefer type checks over null checks.