calling whenDefined with invalid name should throw SyntaxError

third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js

 function create_window_in_test(t) {
   let p = new Promise((resolve) => {
     let f = document.createElement('iframe');
     f.srcdoc = '';
     f.onload = (event) => {
       let w = f.contentWindow;
       t.add_cleanup(() => f.remove());
       resolve(w);
     };
     document.body.appendChild(f);
   });
   return p;
 }
 
 function test_with_window(f, name) {
   promise_test((t) => {
     return create_window_in_test(t)
     .then((w) => {
       f(w);
     });
   }, name);
 }
+
+function assert_throws_dom_exception(global_context, code, func, description) {
+  let exception;
+  assert_throws(code, () => {
+    try {
+      func.call(this);
+    } catch(e) {
+      exception = e;
+      throw e;
+    }
+  }, description);
+  assert_true(exception instanceof global_context.DOMException, 'DOMException on the appropriate window');
+}