A string representation of this object.
Some classes have a default textual representation,
often paired with a static
parse function (like int.parse).
These classes will provide the textual representation as
their string representation.
Other classes have no meaningful textual representation
that a program will care about.
Such classes will typically override
toString to provide
useful information when inspecting the object,
mainly for debugging or logging.
@override String toString() => '$key: $value';