Throw exception if path can't be watched.

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 f5efb69d6e32c39a45791015521125d089081aee..7893ffb4d8b2c51ffc2526de3212d2ed9c3aab87 100644
--- a/runtime/bin/file_system_watcher.cc
+++ b/runtime/bin/file_system_watcher.cc
@@ -35,7 +35,7 @@ void FUNCTION_NAME(FileSystemWatcher_WatchPath)(Dart_NativeArguments args) {
   bool recursive = DartUtils::GetBooleanValue(Dart_GetNativeArgument(args, 3));
   intptr_t id = FileSystemWatcher::WatchPath(path, events, recursive);
   if (id == -1) {
-    Dart_PropagateError(DartUtils::NewDartOSError());
+    Dart_ThrowException(DartUtils::NewDartOSError());
   } else {
     SetWatcherIdNativeField(watcher, id);
   }