Currently we do not check for @@isConcatSpreadable properly. If the Symbol is
set on the Array.prototype or Object.prototype the current fast paths fail.
This CL adds a fix to globally invalidate a isConcatSpreadable_protector.

Drive-by-fix: use named accessors for context variables

LOG=N
BUG=chromium:542504, v8:903

Review-Url: https://codereview.chromium.org/1409123003
Cr-Commit-Position: refs/heads/master@{#36201}