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

sky/framework/sky-element/sky-element.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" />
 <import src="element-registry.sky" as="registry" />
 <script>
 function parseAttributeSpec(registration, definition) {
   var spec = definition.getAttribute('attributes');
@@ skipping 115 matching lines @@
     this.attached();
     this.isAttached = true;
   }
 
   detachedCallback() {
     this.detached();
     this.isAttached = false;
   }
 
   attributeChangedCallback(name, oldValue, newValue) {
+    if (registry.isExpandableAttribute(name))
+      return;
     this.attributeChanged(name, oldValue, newValue);
     var registration = registry.getRegistration(this.localName);
     var converter = registration.attributes.get(name);
     if (converter) {
       this.notifyPropertyChanged(name, converter(oldValue),
           converter(newValue));
     }
   }
 
   notifyPropertyChanged(name, oldValue, newValue) {
@@ skipping 38 matching lines @@
         binding.setValue(this[name]);
         binding.discardChanges();
       }
     }
     this.dirtyPropertyBindings = null;
   }
 };
 
 module.exports = SkyElement;
 </script>