function objMapKeyVal(obj, func, context) {
  if (!obj) {
    return null;
  }
  var i = 0;
  var ret = {};
  for (var key in obj) {
    if (obj.hasOwnProperty(key)) {
      ret[key] = func.call(context, key, obj[key], i++);
    }
  }
  return ret;
}
Behaves the same as `objMap` but invokes func with the key first, and value
second. Use `objMap` unless you need this special case.
Invokes func as:

  func(key, value, iteration)

@param {?object} obj Object to map keys over
@param {!function} func Invoked for each key/val pair.
@param {?*} context
@return {?object} Result of mapping or null if obj is falsey