[es6] Make sure we call add property when adding a new property

[es6] Make sure we call add property when adding a new property

When setting a property using `super.prop = val` we need to use
add property if we are adding a new property and not set property.

BUG=493566
LOG=N
R=verwaest@chromium.org, dslomov@chromium.org

Review URL: https://codereview.chromium.org/1161073002

Cr-Commit-Position: refs/heads/master@{#28971}