UglifyJS fails to minify library files

I enjoy using react-request in my project, thanks for creating it!

I decided to try the new version 3 today, but UglifyJS fails to minify it because of the let keywords in the CommonJS version of the library.

I think it happens because the babel-env plugin configuration is missing the targets section and newer language features are not transpiled.

I am going to amend my configuration to transpile the new react-request version, but it would be nice to have it transpiled out of the box. Moreover, the majority of bundler configuration docs and tutorials assume the code in node_modules is already transpiled to ES5.