Change filename to indicate it uses module kind of es2018

src/harness/harness.ts

@@ -945,6 +945,7 @@ namespace Harness {
 
         export function getDefaultLibFileName(options: ts.CompilerOptions): string {
             switch (options.target) {
+                case ts.ScriptTarget.ESNext:
                 case ts.ScriptTarget.ES2017:
                     return "lib.es2017.d.ts";
                 case ts.ScriptTarget.ES2016: