Fix KeyboardEvent constructor to default to empty string for keyIdentifier.

sdk/lib/html/templates/html/dartium/impl_KeyboardEvent.darttemplate

Index: sdk/lib/html/templates/html/dartium/impl_KeyboardEvent.darttemplate
diff --git a/sdk/lib/html/templates/html/dartium/impl_KeyboardEvent.darttemplate b/sdk/lib/html/templates/html/dartium/impl_KeyboardEvent.darttemplate
index 6e07fa9d2fa26eb829b30095f6a7e19c0b385af4..fe2660006a008d302811862538d3943e22cc65aa 100644
--- a/sdk/lib/html/templates/html/dartium/impl_KeyboardEvent.darttemplate
+++ b/sdk/lib/html/templates/html/dartium/impl_KeyboardEvent.darttemplate
@@ -8,7 +8,7 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
   factory $CLASSNAME(String type, Window view,
       [bool canBubble = true, bool cancelable = true, 
-      String keyIdentifier = null, int keyLocation = 1, bool ctrlKey = false,
+      String keyIdentifier = "", int keyLocation = 1, bool ctrlKey = false,
       bool altKey = false, bool shiftKey = false, bool metaKey = false,
       bool altGraphKey = false]) {
     final e = document.$dom_createEvent("KeyboardEvent");