update elements and fix some bugs

lib/paper_radio_group.dart

Index: lib/paper_radio_group.dart
diff --git a/lib/paper_radio_group.dart b/lib/paper_radio_group.dart
index 1189ed89763dd9ab2492264e8f74c7874c071af4..20e8af09afcd97ffe0426b36b8f2d366104f050b 100644
--- a/lib/paper_radio_group.dart
+++ b/lib/paper_radio_group.dart
@@ -12,11 +12,16 @@ import 'iron_a11y_keys_behavior.dart';
 import 'iron_selectable.dart';
 import 'paper_radio_button.dart';
 
-/// `paper-radio-group` allows user to select only one radio button from a set.
+/// Material design: [Radio button](https://www.google.com/design/spec/components/selection-controls.html#selection-controls-radio-button)
+///
+/// `paper-radio-group` allows user to select at most one radio button from a set.
 /// Checking one radio button that belongs to a radio group unchecks any
 /// previously checked radio button within the same group. Use
 /// `selected` to get or set the selected radio button.
 ///
+/// The <paper-radio-buttons> inside the group must have the `name` attribute
+/// set.
+///
 /// Example:
 ///
 ///     <paper-radio-group selected="small">
@@ -25,13 +30,25 @@ import 'paper_radio_button.dart';
 ///       <paper-radio-button name="large">Large</paper-radio-button>
 ///     </paper-radio-group>
 ///
-/// See <a href="paper-radio-button.html">paper-radio-button</a> for more
+/// Radio-button-groups can be made optional, and allow zero buttons to be selected:
+///
+///     <paper-radio-group selected="small" allow-empty-selection>
+///       <paper-radio-button name="small">Small</paper-radio-button>
+///       <paper-radio-button name="medium">Medium</paper-radio-button>
+///       <paper-radio-button name="large">Large</paper-radio-button>
+///     </paper-radio-group>
+///
+/// See <a href="paper-radio-button">paper-radio-button</a> for more
 /// information about `paper-radio-button`.
 @CustomElementProxy('paper-radio-group')
 class PaperRadioGroup extends HtmlElement with CustomElementProxyMixin, PolymerBase, IronA11yKeysBehavior, IronSelectableBehavior {
   PaperRadioGroup.created() : super.created();
   factory PaperRadioGroup() => new Element.tag('paper-radio-group');
 
+  /// If true, radio-buttons can be deselected
+  bool get allowEmptySelection => jsElement[r'allowEmptySelection'];
+  set allowEmptySelection(bool value) { jsElement[r'allowEmptySelection'] = value; }
+
   /// Overriden from Polymer.IronSelectableBehavior
   String get attrForSelected => jsElement[r'attrForSelected'];
   set attrForSelected(String value) { jsElement[r'attrForSelected'] = value; }