Lexical declarations ('let' and 'const') are part of ES6. The spec for them in Editor's draft is stable.
The are available in IE11 . Firefox plans to ship them in 35 (Jan 2015) .
Lexical declarations has been available in V8 under --harmony-scoping flag for quite some time.
Our current implementation is up-to-date with the latest ES6 spec.
To mitigate possible web-incompatibility, in particular conflicts with legacy 'const' declarations, we are limiting support for lexical declarations to strict mode code for now. (Sloppy mode support for lexical declarations will be done as a distinct language feature process-wise).