Add Directory.createTemp() to asynchronously create a temporary directory.

runtime/bin/directory.cc

Index: runtime/bin/directory.cc
diff --git a/runtime/bin/directory.cc b/runtime/bin/directory.cc
index 24e1326cbc8f847323686302274d89dec8b4989a..7f01cec3e15c5e03170f7099575ef13a46251763 100644
--- a/runtime/bin/directory.cc
+++ b/runtime/bin/directory.cc
@@ -76,6 +76,20 @@ void FUNCTION_NAME(Directory_Create)(Dart_NativeArguments args) {
 }
 
 
+void FUNCTION_NAME(Directory_CreateTemp)(Dart_NativeArguments args) {
+  Dart_EnterScope();
+  Dart_Handle path = Dart_GetNativeArgument(args, 1);
+  if (Dart_IsString(path)) {
+    char* result = Directory::CreateTemp(DartUtils::GetStringValue(path));
+    Dart_SetReturnValue(args, Dart_NewString(result));
+    free(result);
+  } else {
+    Dart_SetReturnValue(args, Dart_NewString(""));
+  }
+  Dart_ExitScope();
+}
+
+
 void FUNCTION_NAME(Directory_Delete)(Dart_NativeArguments args) {
   Dart_EnterScope();
   Dart_Handle path = Dart_GetNativeArgument(args, 1);