Class bindings are mutable and lexically scoped, with TDZ semantics.
They may not overlap with var bindings in the same scope. This patch
adds tests for those properties.

R=adamk
BUG=v8:3305
LOG=N

Review URL: https://codereview.chromium.org/1254003004

Cr-Commit-Position: refs/heads/master@{#29856}