Fixing dartc error about not using ~/ operator and FF test error.

tests/html/wheelevent_test.dart

Index: tests/html/wheelevent_test.dart
diff --git a/tests/html/wheelevent_test.dart b/tests/html/wheelevent_test.dart
index a9932b9677bb2f416606907cbd5133907819c48f..fed10deb7c84be7294529d11037934f5639b773e 100644
--- a/tests/html/wheelevent_test.dart
+++ b/tests/html/wheelevent_test.dart
@@ -23,11 +23,7 @@ main() {
     wheelEvent = 'mousewheel';
   } else if (userAgent.contains('Firefox')) {
     // FF appears to have recently added support for wheel.
-    if (userAgent.contains('Firefox/17')) {
-      wheelEvent = 'DOMMouseScroll';
-    } else {
-      wheelEvent = 'wheel';
-    }
+    wheelEvent = 'wheel';
   } else if (userAgent.contains('WebKit', 0)) {
     wheelEvent = 'mousewheel';
   }