testIsImportant() {
    const wsNode = document.createTextNode(' \t\r\n');
    assertFalse(
        'White space node is ignorable', editorNode.isImportant(wsNode));
    const textNode = document.createTextNode('Hello');
    assertTrue('Text node is important', editorNode.isImportant(textNode));
    const nbspNode = document.createTextNode('\u00a0');
    assertTrue('Node with nbsp is important', editorNode.isImportant(nbspNode));
    const imageNode = googDom.createElement(TagName.IMG);
    assertTrue('Image node is important', editorNode.isImportant(imageNode));
  }
Test if nodes that should be ignorable return false and nodes that should
not be ignored return true.