Note that this issue is not about dropping support for --target es3.

This came up in a meeting today. Given that tsc.exe now uses ChakraCore to run tsc.js, it's not clear how much value users are getting in being able to run TypeScript on ES3 runtimes.

Reasons we'd do this include

• Being able to use ES5 array methods such as map, filter, reduceLeft, some, and every.
• Being able to create empty objects as maps with Object.create(null) to avoid hasOwnProperty calls for every lookup.
• Using get-accessors for lazily computing position information.
• Being able to use JSON.stringify.