Drop obsolete browser support
Created by: Timer
With the recent release of React 16, React is beginning to depend on more browser features such as Map, Set, and requestAnimationFrame.
This issue proposes dropping support for IE9 and IE10, as they had reached End-of-Life on January 12, 2016.
IE 11 has basic support for Map and Set, enough for React to function:
IE 11 also has support for requestAnimationFrame and cancelAnimationFrame.
fetch is not supported in any version of Internet Explorer, nor is Object.assign() or Promise, so all existing polyfills would have to remain.
This will prevent us from having to add about ~6kb in polyfills, and has the added benefit of increasing our syntax support level to that of IE 11.
This allows users to use let, const, and block level function declarations [in depended upon packages]!