Cleanup: Better helpers, unified line splitting. Also asserting that without
strong mode, the snippets don't produce errors.

Fix: The eval related tests were wrong (we didn't make sure that the error
thrown comes from strong scoping and is not a normal runtime error).

BUG=v8:3956
LOG=N

Review URL: https://codereview.chromium.org/993393002

Cr-Commit-Position: refs/heads/master@{#27153}