Update TodoMVC based on https://github.com/polymer/todomvc

samples/third_party/todomvc/web/lib-elements/polymer_selection.dart

+// Copyright 2013 The Polymer Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+library todomvc.web.lib_elements.polymer_selection;
+
+import 'dart:html';
+import 'package:polymer/polymer.dart';
+
+@CustomTag('polymer-selection')
+class PolymerSelection extends PolymerElement {
+  @observable bool multi = false;
+  @observable final selection = new ObservableList();
+
+  factory PolymerSelection() => new Element.tag('polymer-selection');
+  PolymerSelection.created() : super.created();
+
+  void ready() {
+    clear();
+  }
+
+  void clear() {
+    selection.clear();
+  }
+
+  getSelection() {
+    if (multi) return selection;
+    if (selection.isNotEmpty) return selection[0];
+    return null;
+  }
+
+  bool isSelected(item) => selection.contains(item);
+
+  void setItemSelected(item, bool isSelected) {
+    if (item != null) {
+      if (isSelected) {
+        selection.add(item);
+      } else {
+        selection.remove(item);
+      }
+      // TODO(sjmiles): consider replacing with summary
+      // notifications (asynchronous job)
+      asyncFire("polymer-select", detail:
+          new PolymerSelectEventDetail(isSelected, item));
+    }
+  }
+
+  select(item) {
+    if (multi) {
+      toggle(item);
+    } else if (getSelection() != item) {
+      setItemSelected(getSelection(), false);
+      setItemSelected(item, true);
+    }
+  }
+
+  toggle(item) {
+    setItemSelected(item, !isSelected(item));
+  }
+}
+
+class PolymerSelectEventDetail {
+  final bool isSelected;
+  final item;
+
+  PolymerSelectEventDetail(this.isSelected, this.item);
+}