Shhhhh Microsoft doesn't read the web.... they were completely unaware that ES6 was coming. The same way they were with HTML5 and much of CSS3. If only their employees were allowed to surf the net and see what cool stuff is being invented. Oh well, off to work on my cool ActiveX/VML Flash Shim....
Microsoft has a representative on TC39 (Luke Hoban) who is extremely active. In fact, I usually find that my thoughts on some new feature closely align with his.
Because Microsoft has bet on JavaScript heavily for Windows 8 apps (at least for now), and Windows 8 apps have full support for ES5, they have very good insights about ES5.
ES6 is still "coming". Many features are still in flux, and I don't blame Microsoft for not committing to in-flux ES6 features in the timeline of IE10.
