if (Array.isArray(arr)) {
    // $FlowFixMe[incompatible-call] if `T` is an array, `cb` cannot be called
    arr.forEach(cb, scope);
  }
@param {array} arr an "accumulation" of items which is either an Array or
a single item. Useful when paired with the `accumulate` module. This is a
simple utility that allows us to reason about a collection of items, but
handling the case when there is exactly one item (and we do not need to
allocate an array).
@param {function} cb Callback invoked with each element or a collection.
@param {?} [scope] Scope used as `this` in a callback.