Worked on the flutter/flutter repository to deliver a dynamic environment variable injection feature for Flutter Web builds. Developed a new command-line interface option, --web-define, enabling runtime substitution of environment-specific values directly into web templates such as index.html and flutter_bootstrap.js. This approach streamlined multi-environment deployments by reducing manual edits and supporting multiple variables per command, while safeguarding built-in template variables. The implementation included runtime validation and clear error messaging, with comprehensive documentation and usage examples for scenarios like API_URL and ANALYTICS_ID. Utilized Dart, Flutter, and web development skills to integrate the feature into existing build and run workflows.