Add keyLocation to KeyboardEvent constructor for backwards compatibility

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

Index: tools/dom/templates/html/dartium/impl_KeyboardEvent.darttemplate
diff --git a/tools/dom/templates/html/dartium/impl_KeyboardEvent.darttemplate b/tools/dom/templates/html/dartium/impl_KeyboardEvent.darttemplate
index 0d5ec436fe1c11a24bcba26f79fe361d400398a2..578b645926b4a946bac8b358549fe6af59202072 100644
--- a/tools/dom/templates/html/dartium/impl_KeyboardEvent.darttemplate
+++ b/tools/dom/templates/html/dartium/impl_KeyboardEvent.darttemplate
@@ -6,13 +6,20 @@ part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 
-  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

[Jacob, 2017/05/15 17:12:06]

Nit: end comments with a period. Here and in the other template file.

+      bool ctrlKey: false,
+      bool altKey: false,
+      bool shiftKey: false,
+      bool metaKey: false}) {
     if (view == null) {
       view = window;
     }
+    location ??= keyLocation ?? 1;
     final e = document._createEvent("KeyboardEvent");
     e._initKeyboardEvent(type, canBubble, cancelable, view, "",
         location, ctrlKey, altKey, shiftKey, metaKey);