This is a rollup that will let us keep track of ES6 progress.

• let declarations
• const declarations
• Regular expression u flag
• Regular expression y flag
• Binary literals
• New octal literals
• Arrow functions
• Default parameters
• Rest parameters
• Spread operator
• super references
• Classes
• for-of loops
• Generators
• Object literal property shorthand
• Object literal method shorthand
• Object literal computed properties
• Template strings
• Destructuring
• Unicode code point escapes
• Allow duplicate object literal properties (except __proto__)
• Modules