If a key is a substring of an earlier value, then the earlier value
will unintentionally be clobbered. For example with:
  macro SET_PRIVATE(obj, sym, val) = (obj[sym] = val);
  SET_PRIVATE(iterator, arrayIteratorObjectSymbol, object);
if the mapping is:
  {'val': 'object',
   'obj': 'iterator',
   'sym': 'arrayIteratorObjectSymbol'}
then 'obj' -> 'iterator' will clobber 'val' -> 'object', resulting in
'val' -> 'iteratorect'. To fix this, replace all substitutions
simultaneously.

Patch from Zoe Clifford <zoeclifford@google.com>

Review-Url: https://codereview.chromium.org/2249873004
Cr-Commit-Position: refs/heads/master@{#38665}