Fix Dart/Flutter web integer literals JavaScript overflow error

[limexp]

Fix for #4880
Using BigInt for proper handling integer literals >= 2^53 when building Flutter web

[limexp]

The errors in checks were caused by NuGet failures (file "4_Setup Clang.txt").

Do I need to do anything with it now (blank commit, reopen PR, ...)?

2025-09-10T09:27:10.6749380Z Downloading package from source 'https://community.chocolatey.org/api/v2/'
2025-09-10T09:27:10.7344622Z [NuGet] Error downloading 'llvm.21.1.0' from 'https://community.chocolatey.org/api/v2/package/llvm/21.1.0'.
2025-09-10T09:27:10.7345684Z [NuGet] Response status code does not indicate success: 503 (Service Unavailable: Back-end server is at capacity).
2025-09-10T09:27:10.7707139Z [NuGet] Error downloading 'llvm.21.1.0' from 'https://community.chocolatey.org/api/v2/package/llvm/21.1.0'.
2025-09-10T09:27:10.7708033Z [NuGet] Response status code does not indicate success: 503 (Service Unavailable: Back-end server is at capacity).
2025-09-10T09:27:10.7950756Z llvm not upgraded. An error occurred during installation: