Don't set expression attributes before they're bound.

sky/framework/sky-element/element-registry.sky

 <!--
 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 -->
 <import src="sky-binder.sky" as="binder" />
 <script>
 // TODO(esprehn): It would be nice if these were exposed by the platform so
 // the framework didn't need to hard code a list.
 var globalAttributesNames = new Set([
   'class',
   'contenteditable',
   'dir',
   'id',
   'lang',
   'spellcheck',
   'tabindex',
   'style',
 ]);
 
+function isExpandableAttribute(name) {
+  return name.startsWith('data-') || name.startsWith('on-');
+}
+
+function isGlobalAttribute(name) {
+  if (isExpandableAttribute(name))
+    return true;
+  return globalAttributesNames.has(name);
+}
+
 var attributeConverters = {
   boolean: function(value) {
     if (typeof value == 'string')
       return value == 'true';
     return !!value;
   },
   number: function(value) {
     return Number(value);
   },
   string: function(value) {
@@ skipping 18 matching lines @@
 class ElementRegistration {
   constructor(tagName) {
     this.tagName = tagName;
     this.attributes = new Map();
     this.eventHandlers = new Map();
     this.template = null;
     Object.preventExtensions(this);
   }
 
   allowsAttribute(name) {
-    if (name.startsWith('data-'))
-      return true;
-    if (globalAttributesNames.has(name))
+    if (isGlobalAttribute(name))
       return true;
     if (this.attributes.has(name))
       return true;
     return false;
   }
 
   defineAttribute(name, type) {
     var converter = attributeConverters[type];
 
     if (!converter) {
@@ skipping 41 matching lines @@
 }
 
 function getRegistration(tagName) {
   return registrations.get(tagName);
 }
 
 function checkAttribute(tagName, attrName) {
   var registration = getRegistration(tagName);
 
   if (!registration)
-    return globalAttributesNames.has(attrName);
+    return isGlobalAttribute(attrName);
 
   return registration.allowsAttribute(attrName);
 }
 
 registerElement('img', {
   'width': 'number',
   'height': 'number',
   // TODO(esprehn): Sky probably doesn't want the crossorign attr.
   'crossorigin': 'string',
   'src': 'string',
@@ skipping 20 matching lines @@
 
 registerElement('template', {
   'if': 'string',
   'repeat': 'string',
 });
 
 module.exports = {
   registerElement: registerElement,
   getRegistration: getRegistration,
   checkAttribute: checkAttribute,
+  isGlobalAttribute: isGlobalAttribute,
+  isExpandableAttribute: isExpandableAttribute,
 };
 </script>