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Babel preset for all es2015 plugins.

This is [babel-preset-es2015](http://babeljs.io/docs/plugins/preset-es2015/), minus [modules-commonjs](http://babeljs.io/docs/plugins/transform-es2015-modules-commonjs/), plus [external-helpers](http://babeljs.io/docs/plugins/external-helpers/). Use it wi