port 8d946b9c (r27106).

original commit message:

  [es6] Throw TypeError for computed static prototype property name

  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=

Review URL: https://codereview.chromium.org/1028983002

Cr-Commit-Position: refs/heads/master@{#27356}