[flutter_tools] Make flutter git upstream configurable (#61513)

packages/flutter_tools/lib/src/version.dart

diff --git a/packages/flutter_tools/lib/src/version.dart b/packages/flutter_tools/lib/src/version.dart
index e798a36..796dd81 100644
--- a/packages/flutter_tools/lib/src/version.dart
+++ b/packages/flutter_tools/lib/src/version.dart
@@ -24,7 +24,7 @@
 }
 
 /// The flutter GitHub repository.
-const String _flutterGit = 'https://github.com/flutter/flutter.git';
+String get _flutterGit => globals.platform.environment['FLUTTER_GIT_URL'] ?? 'https://github.com/flutter/flutter.git';
 
 /// Retrieve a human-readable name for a given [channel].
 ///
@@ -259,6 +259,13 @@
         branch: '$_versionCheckRemote/$branch',
         lenient: false,
       );
+    } on VersionCheckError catch (error) {
+      if (globals.platform.environment.containsKey('FLUTTER_GIT_URL')) {
+        globals.logger.printError('Warning: the Flutter git upstream was overriden '
+        'by the environment variable FLUTTER_GIT_URL = $_flutterGit');
+      }
+      globals.logger.printError(error.toString());
+      rethrow;
     } finally {
       await _removeVersionCheckRemoteIfExists();
     }