Move _FileSystemWatcher to IO patch since it extends NativeFieldWrapperClass1.

sdk/lib/_internal/lib/io_patch.dart

Index: sdk/lib/_internal/lib/io_patch.dart
diff --git a/sdk/lib/_internal/lib/io_patch.dart b/sdk/lib/_internal/lib/io_patch.dart
index 0bd68ac8d0d0e9c4e4aa3008636da5c4e120ff0d..3955989cbfdca46d15e06b21ca80e6101fbd9f1b 100644
--- a/sdk/lib/_internal/lib/io_patch.dart
+++ b/sdk/lib/_internal/lib/io_patch.dart
@@ -349,3 +349,9 @@ patch class Stdin {
     throw new UnsupportedError("Stdin.lineMode");
   }
 }
+
+patch class _FileSystemWatcher {
+  patch factory _FileSystemWatcher(String path, int events, bool recursive) {
+    throw new UnsupportedError("_FileSystemWatcher._FileSystemWatcher");
+  }
+}