This CL attempts to set the maybe-assigned flag for variables that are written
to as part of a destructuring or loop header.

For instance, in the following two cases we now mark x as maybe-assigned.

a) [x] = [1];
b) for (x of [1,2,3]) {};

There's more work to do here, this is just a first step.

R=adamk@chromium.org, mstarzinger@chromium.org
BUG=v8:5636

Review-Url: https://codereview.chromium.org/2562443003
Cr-Commit-Position: refs/heads/master@{#41582}