Mark effectively static class as static

PiperOrigin-RevId: 600933030

src/com/google/javascript/jscomp/Es6RewriteModulesToCommonJsModules.java

@@ -125,7 +125,7 @@ private static ModuleRequest create(String specifier, String varName) {
    * Rewrites a single ES6 module into a CommonJS like module designed to be loaded in the
    * compiler's module runtime.
    */
-  private class Rewriter extends AbstractPostOrderCallback {
+  private static class Rewriter extends AbstractPostOrderCallback {
     private @Nullable Node requireInsertSpot;
     private final Node script;
     private final Map<String, LocalQName> exportedNameToLocalQName;