Dojo 2 and ES6

[kitsonk]

Now that many browsers have sufficient support for ES6, we should try to support targeting ES6 for Dojo 2 and then consider including an "ES6" targeted version in our distributions.

Since 1.7, TypeScript allows targeting ES6 with other module formats (e.g. UMD).

Things I think we need to do:

• Build a version of Dojo Loader targeting ES6 and run test suite against it
• Build a version of Dojo Core targeting ES6 (still UMD modules) and run test suite against it
• Update the Gruntfile to allow transparent building of ES6 distributions (for now, we should just have something like grunt dev:es6 and grunt dist:es6 instead of creating multiple outputs.

Both activities should be done in feature branches off of the main repos, so they can be worked on easily by multiple parties.

In particular, we might run into issues with some type conflicts with some of the built ins that we try to shim (especially Symbol). I know @devpaul was doing some work on this for #18, but I think this is a big enough chunk of work to break it out separately.

[kitsonk]

We have done enough research into this to know what we know and we don't know.