Throw exception if path can't be watched.

runtime/bin/file_system_watcher.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "bin/file_system_watcher.h"
 
 #include "bin/builtin.h"
 #include "bin/dartutils.h"
 
 #include "include/dart_api.h"
@@ skipping 17 matching lines @@
   Dart_SetReturnValue(args, Dart_NewBoolean(FileSystemWatcher::IsSupported()));
 }
 
 void FUNCTION_NAME(FileSystemWatcher_WatchPath)(Dart_NativeArguments args) {
   Dart_Handle watcher = Dart_GetNativeArgument(args, 0);
   const char* path = DartUtils::GetStringValue(Dart_GetNativeArgument(args, 1));
   int events = DartUtils::GetIntegerValue(Dart_GetNativeArgument(args, 2));
   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);
   }
   intptr_t socket_id = FileSystemWatcher::GetSocketId(id);
   Dart_SetReturnValue(args, Dart_NewInteger(socket_id));
 }
 
 
 void FUNCTION_NAME(FileSystemWatcher_UnwatchPath)(Dart_NativeArguments args) {
   Dart_Handle watcher = Dart_GetNativeArgument(args, 0);
   intptr_t id;
   GetWatcherIdNativeField(watcher, &id);
   FileSystemWatcher::UnwatchPath(id);
 }
 
 
 void FUNCTION_NAME(FileSystemWatcher_ReadEvents)(Dart_NativeArguments args) {
   Dart_Handle watcher = Dart_GetNativeArgument(args, 0);
   intptr_t id;
   GetWatcherIdNativeField(watcher, &id);
   Dart_Handle handle = FileSystemWatcher::ReadEvents(id);
   ThrowIfError(handle);
   Dart_SetReturnValue(args, handle);
 }
 
 }  // namespace bin
 }  // namespace dart