We used to have a parse error for conflicting property keys. This check
was removed to match ES6 (SpiderMonkey & Chakra already made this change).
Since this check was removed we ended up with a few new cases when
generating code. For example, accessors always generated code even if
those should have been shadowed by a data property.

BUG=v8:3856
LOG=Y
R=adamk, dslomov@chromium.org

Review URL: https://codereview.chromium.org/895693002

Cr-Commit-Position: refs/heads/master@{#26441}