Rewrite file-system-watcher to better handle the different system APIs.

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 d262d7995b9f05ba34c6af739d1c2bbadafbb085..bc87e41b546bb272d7ea6d1e60622ed8ce213732 100644
--- a/sdk/lib/_internal/lib/io_patch.dart
+++ b/sdk/lib/_internal/lib/io_patch.dart
@@ -345,8 +345,9 @@ patch class Stdin {
 }
 
 patch class _FileSystemWatcher {
-  patch factory _FileSystemWatcher(String path, int events, bool recursive) {
-    throw new UnsupportedError("_FileSystemWatcher._FileSystemWatcher");
+  patch static Stream<FileSystemEvent> watch(
+      String path, int events, bool recursive) {
+    throw new UnsupportedError("_FileSystemWatcher.watch");
   }
   patch static bool get isSupported {
     throw new UnsupportedError("_FileSystemWatcher.isSupported");