Add keyLocation to KeyboardEvent constructor for backwards compatibility

tools/dom/templates/html/dart2js/impl_KeyboardEvent.darttemplate

Index: tools/dom/templates/html/dart2js/impl_KeyboardEvent.darttemplate
diff --git a/tools/dom/templates/html/dart2js/impl_KeyboardEvent.darttemplate b/tools/dom/templates/html/dart2js/impl_KeyboardEvent.darttemplate
index 408c641ac195d45c93041ea51d4df7389da3ce9f..d66a2cf57934dc7b743a48ee1ac3dc07306a5e2f 100644
--- a/tools/dom/templates/html/dart2js/impl_KeyboardEvent.darttemplate
+++ b/tools/dom/templates/html/dart2js/impl_KeyboardEvent.darttemplate
@@ -24,13 +24,20 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS
    * creating keyboard events with specific key value contents, see the custom
    * Event [KeyEvent].
    */
-  factory $CLASSNAME(String type,
-      {Window view, bool canBubble: true, bool cancelable: true,
-      int location: 1, bool ctrlKey: false,
-      bool altKey: false, bool shiftKey: false, bool metaKey: false}) {
+  factory $CLASSNAME(String type, {
+      Window view,
+      bool canBubble: true,
+      bool cancelable: true,
+      int location,
+      int keyLocation, // Legacy alias for location
+      bool ctrlKey: false,
+      bool altKey: false,
+      bool shiftKey: false,
+      bool metaKey: false}) {
     if (view == null) {
       view = window;
     }
+    location ??= keyLocation ?? 1;
     KeyboardEvent e = document._createEvent("KeyboardEvent");
     e._initKeyboardEvent(type, canBubble, cancelable, view, "",
         location, ctrlKey, altKey, shiftKey, metaKey);