Primitive Map object of dart is not working in Force

[EasonPai]

In the great example -- force

Instead of using the d3 json method to retrieve data, I use a dart Map object directly, then the Force is not working.

original code:

json("miserables.json").then((graph) {
    force
      ..nodes = graph['nodes']
      ..links = graph['links']
      ..start();

    var link =
        svg.selectAll(".link").data(graph['links']).enter().append("line")
          ..attr["class"] = "link"
          ..styleFn["stroke-width"] = (d) => math.sqrt(d['value']);
....

and I comment out the json method, and set data to Force with a dart Map object like this:

var graph = { 
      "nodes": [
        {"name": "Myriel", "group": 1},
        {"name": "Napoleon", "group": 2}
      ],
      "links": [
        {"source": 0, "target": 1, "value": 1},
        {"source": 1, "target": 2, "value": 8}
      ]
    };
// json("miserables.json").then((graph) {
 force
      ..nodes = graph['nodes']
      ..links = graph['links']
      ..start();
...

the 'graph' data supposed is equivalent by these 2 means, but Force is not working anymore.