The prototype of a class constructor function is read only. When we set
computed property names we were ignoring this and we were overriding the
property.

Since the prototype is the only possible own read only property on the
constructor function object we special case this so we do not have to
check this for every property in the class literal.

BUG=v8:3945
LOG=N
R=mstarzinger@chromium.org, dslomov@chromium.org

Review URL: https://codereview.chromium.org/985643003

Cr-Commit-Position: refs/heads/master@{#27106}