analyzer: mock SDK: add Stopwatch

This class is needed for a planned analyzer plugin for the
flutter/flutter repository.

Change-Id: Ia24619ac689931150d79649ab451dd673bdead83
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/452531
Reviewed-by: Ben Konyi <bkonyi@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
diff --git a/pkg/analyzer/lib/src/test_utilities/mock_sdk.dart b/pkg/analyzer/lib/src/test_utilities/mock_sdk.dart
index 17443b8..3381132 100644
--- a/pkg/analyzer/lib/src/test_utilities/mock_sdk.dart
+++ b/pkg/analyzer/lib/src/test_utilities/mock_sdk.dart
@@ -640,6 +640,8 @@
 
 abstract interface class StackTrace {}
 
+class Stopwatch {}
+
 abstract final class String implements Comparable<String>, Pattern {
   external factory String.fromCharCodes(Iterable<int> charCodes,
       [int start = 0, int? end]);