Make file system watcher compile on Mac OS 106, and add a runtime-call to test if the system suppor…

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 3955989cbfdca46d15e06b21ca80e6101fbd9f1b..fc29425912966587c177893c4fbfdacfdff59cc8 100644
--- a/sdk/lib/_internal/lib/io_patch.dart
+++ b/sdk/lib/_internal/lib/io_patch.dart
@@ -354,4 +354,7 @@ patch class _FileSystemWatcher {
   patch factory _FileSystemWatcher(String path, int events, bool recursive) {
     throw new UnsupportedError("_FileSystemWatcher._FileSystemWatcher");
   }
+  patch static bool get isSupported {
+    throw new UnsupportedError("_FileSystemWatcher.isSupported");
+  }
 }