Graph<E>.fromEdges  constructor 
Creates a AdjacencyListGraph 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 Graph.fromEdges(
  Iterable<Edge<E>> edges, {
  bool directed,
}) = AdjacencyListGraph<E>.fromEdges;