enable es6 module syntax

Summary: This is an updated copy of #1993, which was approved by @vjeux but hasn't been rebased. It whitelists the es6 module syntax and updates the JS Environment docs to match. cc @ide @​hkjorgensenCloses #3175

Reviewed By: @​svcscm

Differential Revision: D2498360

Pulled By: @vjeux

Author: corbt

packager/react-packager/.babelrc

@@ -9,6 +9,7 @@
     "es6.classes",
     "es6.constants",
     "es6.destructuring",
+    "es6.modules",
     "es6.parameters",
     "es6.properties.computed",
     "es6.properties.shorthand",

packager/transformer.js

@@ -37,6 +37,7 @@ function transform(src, filename, options) {
       'es6.classes',
       'es6.constants',
       'es6.destructuring',
+      'es6.modules',
       'es6.parameters',
       'es6.properties.computed',
       'es6.properties.shorthand',