message: fix(modal): missing assistive text
  Fixes missing assistive text on the close button.
diff: * LICENSE file in the root directory of this source tree.
  */
  -import { html, customElement, LitElement } from 'lit-element';
  +import { html, property, customElement, LitElement } from 'lit-element';
  import Close20 from '@carbon/icons/lib/close/20';
  import settings from 'carbon-components/es/globals/js/settings';
  +import ifNonNull from '../../globals/directives/if-non-null';
  import styles from './modal.scss';
  const { prefix } = settings;
  @@ -20,13 +21,20 @@ const { prefix } = settings;
  */
  @customElement(`${prefix}-modal-close-button`)
  class BXModalCloseButton extends LitElement {
  + /**
  + * The assistive text for the button.
  + */
  + @property({ attribute: 'assistive-text' })
  + assistiveText = 'Close';
  +
  createRenderRoot() {
  return this.attachShadow({ mode: 'open', delegatesFocus: true });
  }
  render() {
  + const { assistiveText } = this;
  return html`
  - <button class="${prefix}--modal-close">
  + <button aria-label="${ifNonNull(assistiveText)}" class="${prefix}--modal-close" title="${ifNonNull(assistiveText)}">
  ${Close20({
  class: `${prefix}--modal-close__icon`,
  })}
  