This patch strengthens testing of classes by verifying that the binding
that they export externally follows block scoping, as opposed to var-style
scoping. The tests are based on existing tests for let and const.

R=adamk
LOG=N
BUG=v8:3305

Review URL: https://codereview.chromium.org/1286923002

Cr-Commit-Position: refs/heads/master@{#30140}