Improve frb_dart/ffigen_generated

[fzyzcjy]

Changes

Please list issues fixed by this PR here, using format "Fixes #the-issue-number".

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8ef918d) 98.44% compared to head (31f8205) 99.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1598      +/-   ##
==========================================
+ Coverage   98.44%   99.21%   +0.76%     
==========================================
  Files         343      343              
  Lines       13463    13474      +11     
==========================================
+ Hits        13254    13368     +114     
+ Misses        209      106     -103     

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