[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();
     }

packages/flutter_tools/test/general.shard/version_test.dart

diff --git a/packages/flutter_tools/test/general.shard/version_test.dart b/packages/flutter_tools/test/general.shard/version_test.dart
index 47f1343..2ee7c47 100644
--- a/packages/flutter_tools/test/general.shard/version_test.dart
+++ b/packages/flutter_tools/test/general.shard/version_test.dart
@@ -7,6 +7,7 @@
 import 'package:collection/collection.dart' show ListEquality;
 import 'package:flutter_tools/src/base/context.dart';
 import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/base/process.dart';
 import 'package:flutter_tools/src/base/time.dart';
 import 'package:flutter_tools/src/base/utils.dart';
@@ -609,6 +610,45 @@
       environment: anyNamed('environment'),
     )).called(1);
   });
+
+  testUsingContext('determine uses overridden git url', () {
+    final MockProcessUtils processUtils = MockProcessUtils();
+    when(processUtils.runSync(
+      <String>['git', 'rev-parse', '--abbrev-ref', 'HEAD'],
+      workingDirectory: anyNamed('workingDirectory'),
+      environment: anyNamed('environment'),
+    )).thenReturn(RunResult(ProcessResult(108, 0, 'master', ''), <String>['git', 'fetch']));
+    when(processUtils.runSync(
+      <String>['git', 'fetch', 'https://githubmirror.com/flutter.git', '--tags', '-f'],
+      workingDirectory: anyNamed('workingDirectory'),
+      environment: anyNamed('environment'),
+    )).thenReturn(RunResult(ProcessResult(109, 0, '', ''), <String>['git', 'fetch']));
+    when(processUtils.runSync(
+      <String>['git', 'tag', '--contains', 'HEAD'],
+      workingDirectory: anyNamed('workingDirectory'),
+      environment: anyNamed('environment'),
+    )).thenReturn(
+      RunResult(ProcessResult(110, 0, '', ''),
+      <String>['git', 'tag', '--contains', 'HEAD'],
+    ));
+    when(processUtils.runSync(
+      <String>['git', 'describe', '--match', '*.*.*-*.*.pre', '--first-parent', '--long', '--tags'],
+      workingDirectory: anyNamed('workingDirectory'),
+      environment: anyNamed('environment'),
+    )).thenReturn(RunResult(ProcessResult(111, 0, 'v0.1.2-3-1234abcd', ''), <String>['git', 'describe']));
+
+    GitTagVersion.determine(processUtils, workingDirectory: '.', fetchTags: true);
+
+    verify(processUtils.runSync(
+      <String>['git', 'fetch', 'https://githubmirror.com/flutter.git', '--tags', '-f'],
+      workingDirectory: anyNamed('workingDirectory'),
+      environment: anyNamed('environment'),
+    )).called(1);
+  }, overrides: <Type, Generator>{
+    Platform: () => FakePlatform(environment: <String, String>{
+      'FLUTTER_GIT_URL': 'https://githubmirror.com/flutter.git',
+    }),
+  });
 }
 
 void _expectVersionMessage(String message) {