This reverts commit f578d35b because
(immutable) heap numbers are surprisingly mutable in V8. Someone else
might want to cleanup the mess, otherwise we'll just keep it this way.

TBR=jkummerow@chromium.org
BUG=chromium:454894
LOG=y

Review URL: https://codereview.chromium.org/898973003

Cr-Commit-Position: refs/heads/master@{#26447}