Improve pubspecs files and example projects

[mbfakourii]

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

improve pubspecs files and example projects

Feature / Enhancement Description

I suggest that the pubspecs be improved based on the dart site
Also, example projects need to be improved

[parse-github-assistant]

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