This is a change for ES2015. Date objects have mutable state, so having
a mutable prototype is bad for SES requirements, and it is an
inconsistency from the typical ES2015 class style of objects

BUG=v8:4004
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1317403002

Cr-Commit-Position: refs/heads/master@{#30486}