Run Windows watcher in an isolate.

pkgs/watcher/CHANGELOG.md

@@ -1,5 +1,9 @@
 ## 1.1.5-wip
 
+- `DirectoryWatcher` on Windows watches in a separate Isolate to make buffer
+  exhaustion, "Directory watcher closed unexpectedly", much less likely. The old
+  implementation which does not use a separate Isolate is available as
+  `DirectoryWatcher(path, runInIsolateOnWindows: false)`.
 - Polling watchers now check file sizes as well as "last modified" times, so
   they are less likely to miss changes on platforms with low resolution
   timestamps.

pkgs/watcher/lib/src/directory_watcher.dart

@@ -20,6 +20,8 @@ import 'directory_watcher/windows.dart';
 /// the message "Directory watcher closed unexpectedly" on the event stream. The
 /// code using the watcher needs to do additional work to account for the
 /// dropped events, for example by recomputing interesting files from scratch.
+/// By default, the watcher is started in a separate isolate to make this less
+/// likely. Pass `runInIsolateOnWindows = false` to not launch an isolate.
 abstract class DirectoryWatcher implements Watcher {
   /// The directory whose contents are being monitored.
   @Deprecated('Expires in 1.0.0. Use DirectoryWatcher.path instead.')
@@ -35,7 +37,11 @@ abstract class DirectoryWatcher implements Watcher {
   /// shorter will give more immediate feedback at the expense of doing more IO
   /// and higher CPU usage. Defaults to one second. Ignored for non-polling
   /// watchers.
-  factory DirectoryWatcher(String directory, {Duration? pollingDelay}) {
+  ///
+  /// On Windows, pass [runInIsolateOnWindows] `false` to not run the watcher
+  /// in a separate isolate to reduce buffer exhaustion failures.
+  factory DirectoryWatcher(String directory,
+      {Duration? pollingDelay, bool runInIsolateOnWindows = true}) {
     if (FileSystemEntity.isWatchSupported) {
       var customWatcher = createCustomDirectoryWatcher(
         directory,
@@ -44,7 +50,10 @@ abstract class DirectoryWatcher implements Watcher {
       if (customWatcher != null) return customWatcher;
       if (Platform.isLinux) return LinuxDirectoryWatcher(directory);
       if (Platform.isMacOS) return MacOSDirectoryWatcher(directory);
-      if (Platform.isWindows) return WindowsDirectoryWatcher(directory);
+      if (Platform.isWindows) {
+        return WindowsDirectoryWatcher(directory,
+            runInIsolate: runInIsolateOnWindows);
+      }
     }
     return PollingDirectoryWatcher(directory, pollingDelay: pollingDelay);
   }