Added SidebarHeaderDropdownButton to stop console warning (#5959)

webapp/components/sidebar_header_dropdown_button.jsx

@@ -0,0 +1,29 @@
+// Copyright (c) 2015 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import React from 'react';
+
+import Constants from 'utils/constants.jsx';
+
+export default class SidebarHeaderDropdownButton extends React.PureComponent {
+    static propTypes = {
+        bsRole: React.PropTypes.oneOf(['toggle']).isRequired, // eslint-disable-line react/no-unused-prop-types
+        onClick: React.PropTypes.func.isRequired
+    };
+
+    render() {
+        return (
+            <a
+                href='#'
+                className='sidebar-header-dropdown__toggle'
+                onClick={this.props.onClick}
+            >
+                <span
+                    className='sidebar-header-dropdown__icon'
+                    dangerouslySetInnerHTML={{__html: Constants.MENU_ICON}}
+                />
+            </a>
+        );
+    }
+}
+