Remove AstConstructionVisitor/AstNullVisitor

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/679943004

Cr-Commit-Position: refs/heads/master@{#25357}
Implement 'setVariableValue' for debugger block scopes.

R=aandrey@chromium.org, rossberg@chromium.org, yurys@chromium.org
BUG=v8:3690
LOG=N

Review URL: https://codereview.chromium.org/732543002

Cr-Commit-Position: refs/heads/master@{#25358}
Extend typed lowering to cover JSStrictEqual on differing types.

R=rossberg@chromium.org, titzer@chromium.org
TEST=unittests/JSTypedLoweringTest.JSStrictEqualWithTheHole

Review URL: https://codereview.chromium.org/722223003

Cr-Commit-Position: refs/heads/master@{#25359}