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

runtime/bin/file_system_watcher.cc

Index: runtime/bin/file_system_watcher.cc
diff --git a/runtime/bin/file_system_watcher.cc b/runtime/bin/file_system_watcher.cc
index aedefab2869ee9ca06979f8aded9aec250890272..f5efb69d6e32c39a45791015521125d089081aee 100644
--- a/runtime/bin/file_system_watcher.cc
+++ b/runtime/bin/file_system_watcher.cc
@@ -24,6 +24,9 @@ void GetWatcherIdNativeField(Dart_Handle watcher, intptr_t* id) {
   ThrowIfError(Dart_GetNativeInstanceField(watcher, kWatcherNativeField, id));
 }
 
+void FUNCTION_NAME(FileSystemWatcher_IsSupported)(Dart_NativeArguments args) {
+  Dart_SetReturnValue(args, Dart_NewBoolean(FileSystemWatcher::IsSupported()));
+}
 
 void FUNCTION_NAME(FileSystemWatcher_WatchPath)(Dart_NativeArguments args) {
   Dart_Handle watcher = Dart_GetNativeArgument(args, 0);