refactor: Buttons will always use a button element (#3828)

BREAKING CHANGE: button component will always use a button element.

src/js/button.js

@@ -16,8 +16,9 @@ class Button extends ClickableComponent {
   /**
    * Create the `Button`s DOM element.
    *
-   * @param {string} [tag=button]
-   *        Element's node type. e.g. 'button'
+   * @param {string} [tag="button"]
+   *        The element's node type. This argument is IGNORED: no matter what
+   *        is passed, it will always create a `button` element.
    *
    * @param {Object} [props={}]
    *        An object of properties that should be set on the element.
@@ -28,25 +29,13 @@ class Button extends ClickableComponent {
    * @return {Element}
    *         The element that gets created.
    */
-  createEl(tag = 'button', props = {}, attributes = {}) {
+  createEl(tag, props = {}, attributes = {}) {
+    tag = 'button';
+
     props = assign({
       className: this.buildCSSClass()
     }, props);
 
-    if (tag !== 'button') {
-      log.warn(`Creating a Button with an HTML element of ${tag} is deprecated; use ClickableComponent instead.`);
-
-      // Add properties for clickable element which is not a native HTML button
-      props = assign({
-        tabIndex: 0
-      }, props);
-
-      // Add ARIA attributes for clickable element which is not a native HTML button
-      attributes = assign({
-        role: 'button'
-      }, attributes);
-    }
-
     // Add attributes for button element
     attributes = assign({