Fully polyfill KeyEvent so that you can programmatically create your own "keyboard" events.

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

Index: tools/dom/templates/html/dart2js/html_dart2js.darttemplate
diff --git a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
index 67490a8190a13e459be3e9f20884c0d6e5908698..0c19623af2c9b5412c2c95807d7ec3c9b20cfac1 100644
--- a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
+++ b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
@@ -49,9 +49,11 @@ import 'dart:web_sql';
 import 'dart:_js_helper' show
     convertDartClosureToJS, Creates, JavaScriptIndexingBehavior,
     JSName, Null, Returns,
-    findDispatchTagForInterceptorClass, setNativeSubclassDispatchRecord;
+    findDispatchTagForInterceptorClass, setNativeSubclassDispatchRecord,
+    makeLeafDispatchRecord;
 import 'dart:_interceptors' show
-    Interceptor, JSExtendableArray, findInterceptorConstructorForType;
+    Interceptor, JSExtendableArray, findInterceptorConstructorForType,
+    setDispatchProperty;
 import 'dart:_isolate_helper' show IsolateNatives;
 import 'dart:_foreign_helper' show JS;