Optimize add/set/delete operations for string keys in Maps and Sets

Review URL: https://codereview.chromium.org/773993002

Cr-Commit-Position: refs/heads/master@{#25668}
Add js-perf-test for ES6 class default constructor

BUG=v8:3330
LOG=N
TBR=dslomov@chromium.org

Review URL: https://codereview.chromium.org/754523004

Cr-Commit-Position: refs/heads/master@{#25669}
Revert of Optimize add/set/delete operations for string keys in Maps and Sets (patchset #5 id:70001 of https://codereview.chromium.org/773993002/)

Reason for revert:
Caused test failures in mjsunit/es6/collections with --deopt-every-n-times=1

Original issue's description:
> Optimize add/set/delete operations for string keys in Maps and Sets

TBR=dslomov@chromium.org,arv@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/777233003

Cr-Commit-Position: refs/heads/master@{#25670}