With this CL repeated values in elements are combined into a single printout with a range.

BEFORE:
- elements = {
  0: <undefined>
  1: <undefined>
  2: <the_hole>
}

AFTER:
 - elements = {
       0-1: <undefined>
         2: <the_hole>
}

BUG=

Review-Url: https://codereview.chromium.org/2169143003
Cr-Commit-Position: refs/heads/master@{#38069}