Example: 

    const menu = (
      <Menu>
        <MenuItem>An Item</MenuItem>
      </Menu>
    );

    <SplitButton overlay={menu}>Dropdown</SplitButton>

Example (Disabled):

    const menu = (
      <Menu>
        <MenuItem>An Item</MenuItem>
      </Menu>
    );

    <SplitButton disabled={true} overlay={menu}>Dropdown</SplitButton>
