calling whenDefined with invalid name should throw SyntaxError

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

Index: third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js
diff --git a/third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js b/third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js
index 885677d54cd183c6226c7dd07a57753aaf3ec9f1..c4a9e6c5ad706f69b376873bdf97bd5e735f8ef4 100644
--- a/third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js
+++ b/third_party/WebKit/LayoutTests/custom-elements/spec/resources/custom-elements-helpers.js
@@ -20,3 +20,16 @@ function test_with_window(f, name) {
     });
   }, 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');
+}