fixed @import handling issue

lib/single-file/single-file-core.js

@@ -704,7 +704,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 						importedStylesheetContent = DomUtil.wrapMediaQuery(importedStylesheetContent, match.media);
 						if (stylesheetContent.indexOf(cssImport) != -1) {
 							importedStylesheetContent = DomProcessorHelper.resolveStylesheetURLs(importedStylesheetContent, styleSheetUrl);
-							DomProcessorHelper.resolveImportURLs(importedStylesheetContent, styleSheetUrl, options);
+							importedStylesheetContent = await DomProcessorHelper.resolveImportURLs(importedStylesheetContent, styleSheetUrl, options);
 							stylesheetContent = stylesheetContent.replace(cssImport, importedStylesheetContent);
 						}
 					}