Unexpected Token after const
You can't define a const in the class body like that; it should be moved into a method.
render() { const title = ( <h3>Panel title</h3> ); // ...}
Apparently, this is called "Public Class Field Syntax" and is already available in babel as the plugin, babel-plugin-transform-class-properties. I have not tried it as yet though.
Additional reference is reactjs.org events guide