[package_info_plus]: Use Flutter's compile-time build name/number on web instead of version.json

[Ortes]

Plugin

package_info_plus

Use case

On the web, PackageInfo.fromPlatform() reads version / buildNumber from version.json, fetched from the server at runtime. That file describes the currently deployed bundle, not the one running. A client on a stale cached bundle therefore reports the new version, so update gates and "running version" displays fail for exactly the users they exist for (#2675, closed as not planned). The fetch can also fail outright (#456, #3028).

Since Flutter 3.47.0 (flutter/flutter#187935), flutter_tools injects the app's pubspec version (or --build-name / --build-number) as the dart-defines FLUTTER_BUILD_NAME / FLUTTER_BUILD_NUMBER, exposed as appBuildName / appBuildNumber in package:flutter/services.dart. They are compiled into the bundle, so they always describe the code that is executing.

Proposal

In the web implementation, prefer the compile-time values when present and keep version.json as the fallback:

const _buildName = bool.hasEnvironment('FLUTTER_BUILD_NAME')
    ? String.fromEnvironment('FLUTTER_BUILD_NAME')
    : null;
const _buildNumber = bool.hasEnvironment('FLUTTER_BUILD_NUMBER')
    ? String.fromEnvironment('FLUTTER_BUILD_NUMBER')
    : null;

Reading the defines through bool.hasEnvironment (instead of importing appBuildName) keeps the current minimum Flutter version: older SDKs just don't define them, and behaviour is unchanged. On native platforms nothing changes.

This fixes #2675 transparently for every consumer, including SDKs that derive their release from package_info_plus (for example sentry_flutter's release name).

PR: #3990 (replaces the opt-in approach of #3874, which I closed once the Flutter side landed).