Supporting non-SSVs in CustomEvent.detail

tests/html/event_customevent_test.dart

Index: tests/html/event_customevent_test.dart
diff --git a/tests/html/event_customevent_test.dart b/tests/html/event_customevent_test.dart
index f7740e816288112f9099ee0d692c7fb5ce887f52..42e3031d2f878e0f4a0494454a825ddb5d385fdc 100644
--- a/tests/html/event_customevent_test.dart
+++ b/tests/html/event_customevent_test.dart
@@ -8,6 +8,12 @@ import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
+class DartPayloadData {
+  final dartValue;
+
+  DartPayloadData(this.dartValue);
+}
+
 main() {
   useHtmlConfiguration();
 
@@ -46,4 +52,31 @@ main() {
 
     expect(fired, isTrue);
   });
+
+  test('custom events to JS', () {
+    var scriptContents = '''
+      window.addEventListener('dart_custom_event', function(e) {
+        if (e.detail == 'dart_message') {
+          e.preventDefault();
+        }
+      }, false);''';
+
+    document.body.append(new ScriptElement()..text = scriptContents);
+
+    var event = new CustomEvent('dart_custom_event', detail: 'dart_message');
+    window.dispatchEvent(event);
+    expect(event.defaultPrevented, isTrue);
+  });
+
+  test('custom data to Dart', () {
+    var data = new DartPayloadData(666);
+    var event = new CustomEvent('dart_custom_data_event', detail: data);
+
+    var future = window.on['dart_custom_data_event'].first.then((_) {
+      expect(event.detail.dartValue, 666);
+    });
+
+    document.body.dispatchEvent(event);
+    return future;
+  });
 }