fixes #27586, prefer context type in generic inference

tests/lib_strong/html/event_customevent_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library EventCustomEventTest;
 
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import 'dart:js' as js;
@@ skipping 26 matching lines @@
   test('custom events from JS', () {
     var scriptContents = '''
       var event = document.createEvent("CustomEvent");
       event.initCustomEvent("js_custom_event", true, true, {type: "detail"});
       window.dispatchEvent(event);
     ''';
 
     var fired = false;
     window.on['js_custom_event'].listen((ev) {
       fired = true;
-      expect(ev.detail, {'type': 'detail'});
+      expect((ev as CustomEvent).detail, {'type': 'detail'});
     });
 
     var script = new ScriptElement();
     script.text = scriptContents;
     document.body.append(script);
 
     expect(fired, isTrue);
   });
 
   test('custom events to JS', () {
@@ skipping 19 matching lines @@
     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;
   });
 }