Prefer Dart_SetReturnValue over Dart_PropagateError when it makes sense.

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 "bin/utils.h"
 
 #include "include/dart_api.h"
 
 namespace dart {
 namespace bin {
 
 void FUNCTION_NAME(FileSystemWatcher_IsSupported)(Dart_NativeArguments args) {
   Dart_SetReturnValue(args, Dart_NewBoolean(FileSystemWatcher::IsSupported()));
 }
 
 
 void FUNCTION_NAME(FileSystemWatcher_InitWatcher)(Dart_NativeArguments args) {
   intptr_t id = FileSystemWatcher::Init();
   if (id >= 0) {
     Dart_SetReturnValue(args, Dart_NewInteger(id));
   } else {
     OSError os_error;
-    Dart_Handle error = DartUtils::NewDartOSError(&os_error);
-    if (Dart_IsError(error)) Dart_PropagateError(error);
-    Dart_ThrowException(error);
+    Dart_ThrowException(DartUtils::NewDartOSError(&os_error));
   }
 }
 
 
 void FUNCTION_NAME(FileSystemWatcher_CloseWatcher)(Dart_NativeArguments args) {
   intptr_t id = DartUtils::GetIntptrValue(Dart_GetNativeArgument(args, 0));
   FileSystemWatcher::Close(id);
 }
 
 
@@ skipping 28 matching lines @@
 
 void FUNCTION_NAME(FileSystemWatcher_GetSocketId)(Dart_NativeArguments args) {
   intptr_t id = DartUtils::GetIntptrValue(Dart_GetNativeArgument(args, 0));
   intptr_t path_id = DartUtils::GetIntptrValue(Dart_GetNativeArgument(args, 1));
   int socket_id = FileSystemWatcher::GetSocketId(id, path_id);
   Dart_SetReturnValue(args, Dart_NewInteger(socket_id));
 }
 
 }  // namespace bin
 }  // namespace dart