Browser support for Safari < 9 and any IE

[acusti]

Object.assign isn’t supported in any version of desktop or mobile Safari less than v9 and any IE, as well as any android browser (reference), which makes the component break in those browsers. The transform-object-assign babel plugin is a lightweight way to make it compatible (defaults to Object.assign, else uses a polyfill which babel already adds to your transpiled files; see the _extends helper).