Pin the version of flutter/plugins to test against (#71166)

dev/bots/test.dart

diff --git a/dev/bots/test.dart b/dev/bots/test.dart
index c0c732d..61d4383 100644
--- a/dev/bots/test.dart
+++ b/dev/bots/test.dart
@@ -7,6 +7,8 @@
 import 'dart:io';
 import 'dart:math' as math;
 
+import 'package:file/file.dart' as fs;
+import 'package:file/local.dart';
 import 'package:googleapis/bigquery/v2.dart' as bq;
 import 'package:googleapis_auth/auth_io.dart' as auth;
 import 'package:http/http.dart' as http;
@@ -37,6 +39,7 @@
 final String pubCache = path.join(flutterRoot, '.pub-cache');
 final String toolRoot = path.join(flutterRoot, 'packages', 'flutter_tools');
 final String engineVersionFile = path.join(flutterRoot, 'bin', 'internal', 'engine.version');
+final String flutterPluginsVersionFile = path.join(flutterRoot, 'bin', 'internal', 'flutter_plugins.version');
 
 String get platformFolderName {
   if (Platform.isWindows)
@@ -861,6 +864,25 @@
   await _stopChromeDriver();
 }
 
+/// Returns the commit hash of the flutter/plugins repository that's rolled in.
+///
+/// The flutter/plugins repository is a downstream dependency, it is only used
+/// by flutter/flutter for testing purposes, to assure stable tests for a given
+/// flutter commit the flutter/plugins commit hash to test against is coded in
+/// the bin/internal/flutter_plugins.version file.
+///
+/// The `filesystem` parameter specified filesystem to read the plugins version file from.
+/// The `pluginsVersionFile` parameter allows specifying an alternative path for the
+/// plugins version file, when null [flutterPluginsVersionFile] is used.
+Future<String> getFlutterPluginsVersion({
+  fs.FileSystem fileSystem = const LocalFileSystem(),
+  String pluginsVersionFile,
+}) async {
+  final File versionFile = fileSystem.file(pluginsVersionFile ?? flutterPluginsVersionFile);
+  final String versionFileContents = await versionFile.readAsString();
+  return versionFileContents.trim();
+}
+
 /// Executes the test suite for the flutter/plugins repo.
 Future<void> _runFlutterPluginsTests() async {
   Future<void> runAnalyze() async {
@@ -877,6 +899,17 @@
       ],
       workingDirectory: checkout.path,
     );
+    final String pluginsCommit = await getFlutterPluginsVersion();
+    await runCommand(
+      'git',
+      <String>[
+        '-c',
+        'core.longPaths=true',
+        'checkout',
+        pluginsCommit,
+      ],
+      workingDirectory: checkout.path,
+    );
     await runCommand(
       pub,
       <String>[