Fall back to PollingDirectoryWatcher if FileSystemEntity:watch is not supported.

pkg/watcher/lib/src/directory_watcher.dart

Index: pkg/watcher/lib/src/directory_watcher.dart
diff --git a/pkg/watcher/lib/src/directory_watcher.dart b/pkg/watcher/lib/src/directory_watcher.dart
index 27dbb3f5101f56e5bcb8fa08f5b2d4a177362676..605eaea48f95dc4ca62364234dc3ee5275000280 100644
--- a/pkg/watcher/lib/src/directory_watcher.dart
+++ b/pkg/watcher/lib/src/directory_watcher.dart
@@ -55,9 +55,11 @@ abstract class DirectoryWatcher {
   /// and higher CPU usage. Defaults to one second. Ignored for non-polling
   /// watchers.
   factory DirectoryWatcher(String directory, {Duration pollingDelay}) {
-    if (Platform.isLinux) return new LinuxDirectoryWatcher(directory);
-    if (Platform.isMacOS) return new MacOSDirectoryWatcher(directory);
-    if (Platform.isWindows) return new WindowsDirectoryWatcher(directory);
+    if (FileSystemEntity.isWatchSupported) {
+      if (Platform.isLinux) return new LinuxDirectoryWatcher(directory);
+      if (Platform.isMacOS) return new MacOSDirectoryWatcher(directory);
+      if (Platform.isWindows) return new WindowsDirectoryWatcher(directory);
+    }
     return new PollingDirectoryWatcher(directory, pollingDelay: pollingDelay);
   }
 }