update elements and fix some bugs

lib/paper_checkbox.dart

 // DO NOT EDIT: auto-generated with `pub run custom_element_apigen:update`
 
 /// Dart API for the polymer element `paper_checkbox`.
 @HtmlImport('paper_checkbox_nodart.html')
 library polymer_elements.lib.src.paper_checkbox.paper_checkbox;
 
 import 'dart:html';
 import 'dart:js' show JsArray, JsObject;
 import 'package:web_components/web_components.dart';
 import 'package:polymer_interop/polymer_interop.dart';
+import 'paper_checked_element_behavior.dart';
 import 'paper_inky_focus_behavior.dart';
 import 'iron_button_state.dart';
 import 'iron_a11y_keys_behavior.dart';
 import 'iron_control_state.dart';
+import 'paper_ripple_behavior.dart';
 import 'iron_checked_element_behavior.dart';
 import 'iron_form_element_behavior.dart';
 import 'iron_validatable_behavior.dart';
 import 'paper_ripple.dart';
 import 'default_theme.dart';
 import 'color.dart';
 
+/// Material design: [Checkbox](https://www.google.com/design/spec/components/selection-controls.html#selection-controls-checkbox)
+///
 /// `paper-checkbox` is a button that can be either checked or unchecked.  User
 /// can tap the checkbox to check or uncheck it.  Usually you use checkboxes
 /// to allow user to select multiple options from a set.  If you have a single
 /// ON/OFF option, avoid using a single checkbox and use `paper-toggle-button`
 /// instead.
 ///
 /// Example:
 ///
 ///     <paper-checkbox>label</paper-checkbox>
 ///
 ///     <paper-checkbox checked> label</paper-checkbox>
 ///
 /// ### Styling
 ///
 /// The following custom properties and mixins are available for styling:
 ///
 /// Custom property | Description | Default
 /// ----------------|-------------|----------
 /// `--paper-checkbox-unchecked-background-color` | Checkbox background color when the input is not checked | `transparent`
 /// `--paper-checkbox-unchecked-color` | Checkbox border color when the input is not checked | `--primary-text-color`
 /// `--paper-checkbox-unchecked-ink-color` | Selected/focus ripple color when the input is not checked | `--primary-text-color`
 /// `--paper-checkbox-checked-color` | Checkbox color when the input is checked | `--default-primary-color`
 /// `--paper-checkbox-checked-ink-color` | Selected/focus ripple color when the input is checked | `--default-primary-color`
 /// `--paper-checkbox-checkmark-color` | Checkmark color | `white`
 /// `--paper-checkbox-label-color` | Label color | `--primary-text-color`
+/// `--paper-checkbox-label-spacing` | Spacing between the label and the checkbox | `8px`
 /// `--paper-checkbox-error-color` | Checkbox color when invalid | `--google-red-500`
 @CustomElementProxy('paper-checkbox')
-class PaperCheckbox extends HtmlElement with CustomElementProxyMixin, PolymerBase, IronA11yKeysBehavior, IronButtonState, IronControlState, PaperInkyFocusBehavior, IronFormElementBehavior, IronValidatableBehavior, IronCheckedElementBehavior {
+class PaperCheckbox extends HtmlElement with CustomElementProxyMixin, PolymerBase, IronA11yKeysBehavior, IronButtonState, IronControlState, PaperRippleBehavior, PaperInkyFocusBehavior, IronFormElementBehavior, IronValidatableBehavior, IronCheckedElementBehavior, PaperCheckedElementBehavior {
   PaperCheckbox.created() : super.created();
   factory PaperCheckbox() => new Element.tag('paper-checkbox');
 
   /// Fired when the checked state changes.
   String get ariaActiveAttribute => jsElement[r'ariaActiveAttribute'];
   set ariaActiveAttribute(String value) { jsElement[r'ariaActiveAttribute'] = value; }
-
-  /// Update the checkbox aria-label. This is a temporary workaround not
-  /// being able to observe changes in <content>
-  /// (see: https://github.com/Polymer/polymer/issues/1773)
-  ///
-  /// Call this if you manually change the contents of the checkbox
-  /// and want the aria-label to match the new contents.
-  updateAriaLabel() =>
-      jsElement.callMethod('updateAriaLabel', []);
 }