AdjacencyListGraph<E>.fromEdges  constructor 
Creates an adjacency list graph from edges.
If directed is true, which is the default, the graph is directed, i.e.
an edge has an explicit source and a target node. If directed is
false, the graph is undirected, and the inverse of an edge is also an
edge.
Implementation
factory AdjacencyListGraph.fromEdges(
  Iterable<Edge<E>> edges, {
  bool directed = true,
}) {
  final graph = AdjacencyListGraph<E>(directed: directed);
  graph.addEdges(edges);
  return graph;
}