js/libs/ember-1.5.1.js
function dependentKeysDidChange(obj, depKey, meta) {
  if (obj.isDestroying) { return; }

  var seen = DID_SEEN, top = !seen;
  if (top) { seen = DID_SEEN = {}; }
  iterDeps(propertyDidChange, obj, depKey, seen, meta);
  if (top) { DID_SEEN = null; }
}
function dependentKeysDidChange(obj, depKey, meta) {
  if (obj.isDestroying) { return; }

  var seen = DID_SEEN, top = !seen;
  if (top) { seen = DID_SEEN = {}; }
  iterDeps(propertyDidChange, obj, depKey, seen, meta);
  if (top) { DID_SEEN = null; }
}
called whenever a property has just changed to update dependent keys