Switch to using HashChangeEvent constructor instead of initHashChangeEvent

tools/dom/templates/html/impl/impl_HashChangeEvent.darttemplate

Index: tools/dom/templates/html/impl/impl_HashChangeEvent.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HashChangeEvent.darttemplate b/tools/dom/templates/html/impl/impl_HashChangeEvent.darttemplate
index b9fd424b05028e23ed89781a4b4e97bbc3b865a7..bf7bd07a02341b41c7dc3771afde663abbb6a515 100644
--- a/tools/dom/templates/html/impl/impl_HashChangeEvent.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HashChangeEvent.darttemplate
@@ -9,9 +9,24 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS
   factory $CLASSNAME(String type,
       {bool canBubble: true, bool cancelable: true, String oldUrl,
       String newUrl}) {
+
+$if DART2JS
+    var options = {
+      'canBubble' : canBubble,
+      'cancelable' : cancelable,
+      'oldURL': oldUrl,
+      'newURL': newUrl,
+    };
+    return JS('HashChangeEvent', 'new HashChangeEvent(#, #)',
+        type, convertDartToNative_Dictionary(options));
+$else
+    // TODO(alanknight): This is required while we're on Dartium 39, but will need
+    // to look like dart2js with later versions when initHashChange is removed.
     var event = document._createEvent("HashChangeEvent");
     event._initHashChangeEvent(type, canBubble, cancelable, oldUrl, newUrl);
     return event;
+$endif
   }
+
 $!MEMBERS
 }