Optimize Object.seal and Object.preventExtensions

They both now run fast (due to utilizing transitions instead of always
creating new maps) and sealed or non-extensible objects can stay in
fast mode after transitioning.

This almost entirely reuses the code for transitioning objects
frozen by Object.freeze(), with the added benefit of freeing
up a bit on the map (we no longer keep track of frozen-ness,
as that bit wasn't used for anything interesting).

BUG=v8:3662,chromium:115960
LOG=y

Review URL: https://codereview.chromium.org/776143005

Cr-Commit-Position: refs/heads/master@{#25759}
Update strict mode function declaration tests before block scoping.

R=arv@chromium.org,marja@chromium.org
BUG=v8:2198
LOG=N

Review URL: https://codereview.chromium.org/788143004

Cr-Commit-Position: refs/heads/master@{#25760}
Ship ES6 block scoping.

R=rossberg@chromium.org
BUG=v8:2198
LOG=Y

Review URL: https://codereview.chromium.org/792543002

Cr-Commit-Position: refs/heads/master@{#25761}
Implement the `RegExp.prototype.flags` getter

TEST=mjsunit/harmony
BUG=v8:3751
LOG=N

Review URL: https://codereview.chromium.org/770333005

Cr-Commit-Position: refs/heads/master@{#25762}