Add Closure compiler externs and an eventtype enum for paper-checkbox…

…. This is dependent on the externs for paper-radio-button (googlearchive/paper-radio-button#8) and Polymer (Polymer/polymer#769).

eventtype.js

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+/**
+ * @license
+ * Copyright (c) 2014 The Polymer Project Authors. All rights reserved.
+ * This code may only be used under the BSD style license found at
+ * http://polymer.github.io/LICENSE.txt. The complete set of authors may be
+ * found at http://polymer.github.io/AUTHORS.txt. The complete set of
+ * contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt. Code
+ * distributed by Google as part of the polymer project is also subject to an
+ * additional IP rights grant found at http://polymer.github.io/PATENTS.txt.
+ */
+
+goog.provide('PaperCheckboxEventType');
+
+
+/**
+ * Constants for event names fired by paper-checkbox elements.
+ * @enum {string}
+ */
+PaperCheckboxEventType = {
+  // Fired when the checked state changes due to user interaction.
+  CHANGE: 'change',
+  // Fired when the checked state changes.
+  CORE_CHANGE: 'core-change'
+};
+

paper-checkbox.externs.js

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+/**
+ * @fileoverview Closure compiler externs for paper-checkbox. See docs at
+ * https://www.polymer-project.org/docs/elements/paper-elements.html#paper-checkbox.
+ *
+ * @license
+ * Copyright (c) 2014 The Polymer Project Authors. All rights reserved.
+ * This code may only be used under the BSD style license found at
+ * http://polymer.github.io/LICENSE.txt. The complete set of authors may be
+ * found at http://polymer.github.io/AUTHORS.txt. The complete set of
+ * contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt. Code
+ * distributed by Google as part of the polymer project is also subject to an
+ * additional IP rights grant found at http://polymer.github.io/PATENTS.txt.
+ */
+
+
+/** @constructor @extends {PaperRadioButtonElement} */
+var PaperCheckboxElement = function() {};
+