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}
Create optimized versions of the Map/Set clear method

This completes the first round of optimizations for Map and Set.
All non-key-dependent methods now have a Hydrogen version, and
for keyed methods, string versions are optimized.

Review URL: https://codereview.chromium.org/796503002

Cr-Commit-Position: refs/heads/master@{#25763}
Fix crash in V8 during serializing objects requiring alignment.

Review URL: https://codereview.chromium.org/793753002

Cr-Commit-Position: refs/heads/master@{#25764}
[turbofan] Fix typing of typed array loads/stores.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/794113002

Cr-Commit-Position: refs/heads/master@{#25765}
Switch icu repo to icu.git in v8 DEPS.

This ports https://codereview.chromium.org/769413004 to v8
DEPS.

BUG=chromium:438401
LOG=n
TBR=jshin@chromium.org

Review URL: https://codereview.chromium.org/796813002

Cr-Commit-Position: refs/heads/master@{#25766}