refactor: ES5 no longer have css files

Author: alan-agius4

packages/angular_devkit/build_angular/src/angular-cli-files/utilities/index-file/write-index-html.ts

@@ -51,7 +51,7 @@ export function writeIndexHtml({
         deployUrl,
         sri,
         entrypoints: generateEntryPoints({ scripts, styles }),
-        files: filterAndMapBuildFiles(ES5BuildFiles, '.css'),
+        files: filterAndMapBuildFiles(ES2015BuildFiles, '.css'),
         noModuleFiles: filterAndMapBuildFiles(ES5BuildFiles, '.js'),
         moduleFiles: filterAndMapBuildFiles(ES2015BuildFiles, '.js'),
         loadOutputFile: async filePath => {