port 7516fe1e (r39052)

  original commit message:
  The only remaining use of this VariableMode is for the names of sloppy
  named function expressions. This patch instead uses CONST for such
  bindings (just as we do in strict mode) and instead marks those
  Variables specially. During code generation a new helper method,
  Variable::throw_on_const_assignment(), is called to decide whether
  to throw or silently ignore the assignment.

BUG=

Review-Url: https://codereview.chromium.org/2299103002
Cr-Commit-Position: refs/heads/master@{#39059}