Make the stock app demo list view prettier

sky/framework/sky-element.sky

 <!--
 // Copyright 2015 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.
 -->
 <script>
 import "dart:mirrors";
 import "dart:sky";
 
 typedef dynamic _Converter(String value);
 
 final Map<String, _Converter> _kAttributeConverters = {
   'boolean': (String value) {
     return value == 'true';
   },
   'number': (String value) {
-    return double.parse(value);
+    try {
+      return double.parse(value);
+    } catch(_) {
+      return 0.0;
+    }
   },
   'string': (String value) {
     return value == null ? '' : value;
   },
 };
 
 class _Registration {
   final Element template;
   final Map<String, _Converter> attributes = new Map();
 
@@ skipping 123 matching lines @@
     throw new UnsupportedError('@Tagname can only be used on descendants of SkyElement');
 
   String tagName = _getTagName(type);
   Element template = definition.querySelector('template');
 
   document.registerElement(tagName, type);
   _registery[tagName] = new _Registration(template)
                         ..parseAttributeSpec(definition);
 }
 </script>