NonPatternRewrite was called more than once for the same AST
in the case of (computed) key expressions present in object
literals.  As an example, in:

   var x = { [[...42]]: 17 };

the array containing the spread would be desugared first and
then the resulting do-expression would again be desugared.

This could be problematic if a computed key expression contains
large nested array/object literals.

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1645023002

Cr-Commit-Position: refs/heads/master@{#33632}