Run file watcher symlink tests.

pkgs/watcher/test/file_watcher/link_tests.dart

@@ -98,7 +98,7 @@ void linkTests({required bool isNative}) {
 
   test('notifies when a link is moved away', () async {
     await startWatcher(path: 'link.txt');
-    renameFile('link.txt', 'new.txt');
+    renameLink('link.txt', 'new.txt');
 
     // TODO(davidmorgan): reconcile differences.
     if (isNative) {

pkgs/watcher/test/file_watcher/native_test.dart

@@ -10,11 +10,13 @@ import 'package:watcher/src/file_watcher/native.dart';
 
 import '../utils.dart';
 import 'file_tests.dart';
+import 'link_tests.dart';
 import 'startup_race_tests.dart';
 
 void main() {
   watcherFactory = NativeFileWatcher.new;
 
   fileTests(isNative: true);
+  linkTests(isNative: true);
   startupRaceTests(isNative: true);
 }

pkgs/watcher/test/file_watcher/polling_test.dart

@@ -6,12 +6,14 @@ import 'package:watcher/watcher.dart';
 
 import '../utils.dart';
 import 'file_tests.dart';
+import 'link_tests.dart';
 import 'startup_race_tests.dart';
 
 void main() {
   watcherFactory = (file) =>
       PollingFileWatcher(file, pollingDelay: const Duration(milliseconds: 100));
 
   fileTests(isNative: false);
+  linkTests(isNative: false);
   startupRaceTests(isNative: false);
 }

pkgs/watcher/test/utils.dart

@@ -292,6 +292,20 @@ void renameFile(String from, String to) {
       ifAbsent: () => 1);
 }
 
+/// Schedules renaming a link in the sandbox from [from] to [to].
+///
+/// On MacOS and Linux links can also be named with `renameFile`. On Windows,
+/// however, a link must be renamed with this method.
+void renameLink(String from, String to) {
+  Link(p.join(d.sandbox, from)).renameSync(p.join(d.sandbox, to));
+
+  // Make sure we always use the same separator on Windows.
+  to = p.normalize(to);
+
+  _mockFileModificationTimes.update(to, (value) => value + 1,
+      ifAbsent: () => 1);
+}
+
 /// Schedules creating a directory in the sandbox at [path].
 void createDir(String path) {
   Directory(p.join(d.sandbox, path)).createSync();