fix ES2015 feature of getter
fix ES2015 feature of getter

We get a javascript error with this syntax on older browsers.

ipython/ipywidgets,SylvainCorlay/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,cornhundred/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets