description

# update method

V update (K key, V update(V value), { V ifAbsent() })

Updates the value for the provided `key`.

Returns the new value of the key.

If the key is present, invokes `update` with the current value and stores the new value in the map.

If the key is not present and `ifAbsent` is provided, calls `ifAbsent` and adds the key with the returned value to the map.

It's an error if the key is not present and `ifAbsent` is not provided.

## Implementation

    V update(K key, V update(V value), {V ifAbsent()});