port 6ac4de87

original commit message:
  harmony-scoping: make assignment to 'const' a late error.

  Per TC39 Nov 2014 decision. This patch also changes behavior for "legacy const":
  assignments to sloppy const in strict mode is now also a type error. This fixes
  v8:2243 and also brings us in compliance with other engines re assignment to function
  names (see updated webkit test), but might have bigger implications. That change can
  easily be reverted by changing Variable::IsSignallingAssignmentToConst.

BUG=
R=dslomov@chromium.org, weiliang.lin@intel.com

Review URL: https://codereview.chromium.org/763033002

Patch from Chunyang Dai <chunyang.dai@intel.com>.

Cr-Commit-Position: refs/heads/master@{#25532}