update elements and fix some bugs

test/fixtures/x_list.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 @HtmlImport('x_list.html')
 library polymer_elements.test.fixtures.x_list;
 
 import 'package:polymer/polymer.dart';
 import 'package:polymer_elements/iron_flex_layout.dart';
 import 'package:polymer_elements/iron_list.dart';
 import 'package:web_components/web_components.dart';
 
 /// Uses [IronFlexLayout] and [IronList]
 @PolymerRegister('x-list')
 class XList extends PolymerElement {
   XList.created() : super.created();
 
   @property
   List data;
 
   @property
   int itemHeight = 100;
 
   @property
   int listHeight = 300;
+  
+  @property
+  bool pre = false;
 
   get list => $['list'];
 
   @reflectable
-  String computedItemHeight() => 'height: ${itemHeight}px';
+  String computedItemHeight(item) {
+    var css = pre ? 'white-space:pre;' : '';
+    if (item['height'] != null && item['height'] != 0) {
+      css += itemHeight == 0 ? '' : 'height: ${item['height']}px;';
+    } else if (itemHeight != 0) {
+      css += 'height: ${itemHeight}px;';
+    }
+    return css;
+  }
 
   @reflectable
   String computedListHeight(listHeight) => 'height: ${listHeight}px';
 }