feat(package_info_plus): recognize tool-provided version defines as fallbacks

PackageInfoEnvironment now resolves the web version from, in order:
PACKAGE_INFO_PLUS_VERSION (explicit), FLUTTER_BUILD_NAME
(flutter/flutter#187935, injected by flutter_tools like
FLUTTER_APP_FLAVOR), and dart.package.version (dart-lang/sdk#38855).
Once either upstream proposal lands, apps need no configuration at all.

Author: Ortes

packages/package_info_plus/package_info_plus/README.md

@@ -162,6 +162,12 @@ Behaviour per platform:
 - **All other platforms** — delegates to `PackageInfo.fromPlatform()`, which reads the installed
   binary and is already reliable. The defines are not required there.
 
+The accessor also recognizes tool-provided defines as fallbacks, so apps need no configuration at
+all once their build front-end injects the version itself: `FLUTTER_BUILD_NAME` /
+`FLUTTER_BUILD_NUMBER` (proposed in [flutter/flutter#187935](https://github.com/flutter/flutter/pull/187935))
+and `dart.package.version` ([dart-lang/sdk#38855](https://github.com/dart-lang/sdk/issues/38855)).
+The explicit `PACKAGE_INFO_PLUS_*` defines take precedence over both.
+
 `PackageInfo.fromPlatform()` is unchanged; this accessor lives in a separate library that you import
 only when you opt in, so existing builds are unaffected.
 

packages/package_info_plus/package_info_plus/lib/package_info_plus_environment.dart

@@ -17,6 +17,21 @@ const _buildNumber = String.fromEnvironment('PACKAGE_INFO_PLUS_BUILD_NUMBER');
 const _appName = String.fromEnvironment('PACKAGE_INFO_PLUS_APP_NAME');
 const _packageName = String.fromEnvironment('PACKAGE_INFO_PLUS_PACKAGE_NAME');
 
+/// Tool-provided fallbacks, recognized so that apps need no configuration at
+/// all once their build front-end injects the version itself:
+///
+/// * `FLUTTER_BUILD_NAME` / `FLUTTER_BUILD_NUMBER` — proposed in
+///   flutter/flutter#187935 (injected by flutter_tools from the pubspec
+///   `version`, like `FLUTTER_APP_FLAVOR` already is).
+/// * `dart.package.version` — companion proposal for the `dart` CLI
+///   (dart-lang/sdk#38855); carries the verbatim pubspec `version`,
+///   including any `+buildNumber` suffix.
+///
+/// The explicit `PACKAGE_INFO_PLUS_*` defines take precedence over both.
+const _flutterBuildName = String.fromEnvironment('FLUTTER_BUILD_NAME');
+const _flutterBuildNumber = String.fromEnvironment('FLUTTER_BUILD_NUMBER');
+const _dartPackageVersion = String.fromEnvironment('dart.package.version');
+
 /// Holds the compile-time package info and enforces, at compile time, that a
 /// version was provided on web builds.
 ///
@@ -28,7 +43,10 @@ const _packageName = String.fromEnvironment('PACKAGE_INFO_PLUS_PACKAGE_NAME');
 class _CompileTimePackageInfo {
   const _CompileTimePackageInfo()
     : assert(
-        !kIsWeb || _version != '',
+        !kIsWeb ||
+            _version != '' ||
+            _flutterBuildName != '' ||
+            _dartPackageVersion != '',
         'PACKAGE_INFO_PLUS_VERSION must be provided via --dart-define on web '
         'builds. On the web there is no reliable runtime source for the '
         'version of the *running* bundle (version.json is fetched from the '
@@ -38,8 +56,21 @@ class _CompileTimePackageInfo {
         '_PACKAGE_NAME) to your web build.',
       );
 
-  String get version => _version;
-  String get buildNumber => _buildNumber;
+  String get version {
+    if (_version != '') return _version;
+    if (_flutterBuildName != '') return _flutterBuildName;
+    return _dartPackageVersion.split('+').first;
+  }
+
+  String get buildNumber {
+    if (_buildNumber != '') return _buildNumber;
+    if (_flutterBuildNumber != '') return _flutterBuildNumber;
+    if (_dartPackageVersion.contains('+')) {
+      return _dartPackageVersion.split('+').last;
+    }
+    return '';
+  }
+
   String get appName => _appName;
   String get packageName => _packageName;
 }

packages/package_info_plus/package_info_plus/test/package_info_environment_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart' show kIsWeb;
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:package_info_plus/package_info_plus_environment.dart';
@@ -41,4 +42,31 @@ void main() {
     },
     onPlatform: {'browser': const Skip('Web path is compile-time enforced')},
   );
+
+  // Exercises the tool-provided fallback chain on the web path. Run with:
+  //
+  // flutter test test/package_info_environment_test.dart --platform chrome \
+  //   --dart-define=FLUTTER_BUILD_NAME=9.9.9 --dart-define=FLUTTER_BUILD_NUMBER=7
+  //
+  // (No PACKAGE_INFO_PLUS_VERSION: the explicit define would take precedence.)
+  // Note: once flutter/flutter#187935 lands, FLUTTER_BUILD_NAME becomes
+  // tool-reserved and is injected automatically instead of being passed by hand.
+  test(
+    'web path falls back to the tool-provided FLUTTER_BUILD_NAME defines',
+    () async {
+      if (!kIsWeb) return;
+      final info = await PackageInfoEnvironment.packageInfo;
+      expect(info.version, const String.fromEnvironment('FLUTTER_BUILD_NAME'));
+      expect(
+        info.buildNumber,
+        const String.fromEnvironment('FLUTTER_BUILD_NUMBER'),
+      );
+    },
+    skip:
+        const bool.hasEnvironment('PACKAGE_INFO_PLUS_VERSION') ||
+            !const bool.hasEnvironment('FLUTTER_BUILD_NAME')
+        ? 'Requires --dart-define=FLUTTER_BUILD_NAME and no '
+              'PACKAGE_INFO_PLUS_VERSION (see comment above)'
+        : false,
+  );
 }