Make the stock app demo list view prettier

sky/framework/sky-element.sky

Index: sky/framework/sky-element.sky
diff --git a/sky/framework/sky-element.sky b/sky/framework/sky-element.sky
index e1d0a0ea715da04f9ea53e51ecceeb5be2b74d6f..5a4082316a3f08f5a716a48804a695a8c57c08f9 100644
--- a/sky/framework/sky-element.sky
+++ b/sky/framework/sky-element.sky
@@ -14,7 +14,11 @@ final Map<String, _Converter> _kAttributeConverters = {
     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;