chore: Flutter 3 fixes

[dnys1]

Issue #, if available:

Description of changes:
Fixes for Flutter 3/Dart 2.17

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[codecov-commenter]

Codecov Report

Merging #1580 (60d6a2f) into main (2256175) will increase coverage by 9.47%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1580      +/-   ##
==========================================
+ Coverage   36.69%   46.17%   +9.47%     
==========================================
  Files         240      260      +20     
  Lines        8167    10237    +2070     
==========================================
+ Hits         2997     4727    +1730     
- Misses       5170     5510     +340     

Flag	Coverage Δ
android-unit-tests	∅ <ø> (?)
flutter-unit-tests	36.69% <ø> (ø)
ios-unit-tests	83.57% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../example/ios/unit_tests/resources/SchemaData.swift	100.00% <0.00%> (ø)
...xample/ios/unit_tests/FlutterURLSessionTests.swift	0.00% <0.00%> (ø)
...plify_api_ios/example/ios/Runner/AppDelegate.swift	0.00% <0.00%> (ø)
...s/example/ios/unit_tests/GraphQLApiUnitTests.swift	89.50% <0.00%> (ø)
...e/ios/unit_tests/MockAnalyticsCategoryPlugin.swift	15.38% <0.00%> (ø)
.../ios/unit_tests/amplify_flutter_exampleTests.swift	94.59% <0.00%> (ø)
...ify_datastore/example/ios/Runner/AppDelegate.swift	0.00% <0.00%> (ø)
...e/example/ios/unit_tests/GetJsonFromFileUtil.swift	57.14% <0.00%> (ø)
...os/unit_tests/ModelSchemaEquatableExtensions.swift	89.83% <0.00%> (ø)
...y_flutter_ios/example/ios/Runner/AppDelegate.swift	0.00% <0.00%> (ø)
... and 10 more