Version 3.28.21 (based on bleeding_edge revision r22338)

Make `let` usable as an identifier in ES6 sloppy mode (issue 2198).

Support ES6 Map and Set in heap profiler (issue 3368).

Performance and stability improvements on all platforms.

git-svn-id: https://v8.googlecode.com/svn/trunk@22341 ce2b1a6d-e550-0410-aec6-3dcde31c8c00