In short, array literals containing spreads, when used as expressions,
are rewritten using do expressions.  E.g.

    [1, 2, 3, ...x, 4, ...y, 5]

is roughly rewritten as:

    do {
      $R = [1, 2, 3];
      for ($i of x) %AppendElement($R, $i);
      %AppendElement($R, 4);
      for ($j of y) %AppendElement($R, $j);
      %AppendElement($R, 5);
      $R
    }

where $R, $i and $j are fresh temporary variables.

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1564083002

Cr-Commit-Position: refs/heads/master@{#33307}