toDot method
Implementation
String toDot() {
String res = "digraph $name {";
_nodes.forEach((key, value) {
res += '\n $key [label="$value"]';
});
_edges.forEach((elem) {
res += '\n ${elem.a} -> ${elem.b} [label="${elem.label}"]';
});
res += "\n}";
return res;
}