Is there an existing issue for this?

• I have searched the existing issues
• I have read the guide to filing a bug

Use case

For now it supports in Android (Gradle) and iOS(Info.plist) platforms to get --dart-defined variables.
But not supported in Flutter Web (index.html)

Proposal

Replaceable ${DART_DEFINED_VAR} in index.html like:

<meta name="google-signin-client_id" content="${GOOGLE_SIGN_IN_WEB_CLIENT_ID}">
...
<title>${APP_NAME}</title>