ScriptableRenderPipeline/com.unity.shadergraph/Editor/Importers/ShaderSubGraphImporter.cs

using UnityEditor.Experimental.AssetImporters;
using UnityEditor.ShaderGraph;
using UnityEngine;
using System.IO;
using System.Text;

[ScriptedImporter(1, "ShaderSubGraph")]
public class ShaderSubGraphImporter : ScriptedImporter
{
    public override void OnImportAsset(AssetImportContext ctx)
    {
        var textGraph = File.ReadAllText(ctx.assetPath, Encoding.UTF8);
        var graph = JsonUtility.FromJson<SubGraph>(textGraph);

        if (graph == null)
            return;

        var graphAsset = ScriptableObject.CreateInstance<MaterialSubGraphAsset>();
        graphAsset.subGraph = graph;
        ctx.AddObjectToAsset("MainAsset", graphAsset);
        ctx.SetMainObject(graphAsset);
    }
}