refactor(es6): Update DefinePlugin to es2015 (#3632)
refactor(es6): Update DefinePlugin to es2015 (#3632)

* convert DefinePlugin to es2015
pugnascotia/webpack,pugnascotia/webpack,rlugojr/webpack,webpack/webpack,SimenB/webpack,webpack/webpack,webpack/webpack,ts-webpack/webpack,SimenB/webpack,webpack/webpack,nkzawa/webpack,jescalan/webpack,g0ddish/webpack,g0ddish/webpack,EliteScientist/webpack,ts-webpack/webpack,SimenB/webpack,EliteScientist/webpack,pugnascotia/webpack,nkzawa/webpack,cesarandreu/webpack,ts-webpack/webpack,ts-webpack/webpack,NekR/webpack,SimenB/webpack,rlugojr/webpack,jescalan/webpack,NekR/webpack,cesarandreu/webpack,g0ddish/webpack,cesarandreu/webpack,rlugojr/webpack,nkzawa/webpack,jescalan/webpack