Looks like AOT failure. When accessing Node<T>.node directly - everything works fine. If it's wrapped with ReadOnlyCollection<> property, then AOT fails. Throws assert in mono:

Node<Test>[] doesn't implement interface System.Collections.Generic.IEnumerable<Node<T>>
* Assertion: should not be reached at mini-trampolines.c:183

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;

interface ITest
{
void DoStuff();
}

class Test: ITest
{
public void DoStuff()
{
Debug.Log("DoStuff");
}
}

class Node<T>
{
public Node<T>[] nodes = new Node<T>[1];

public ReadOnlyCollection<Node<T>> Nodes { get { return new ReadOnlyCollection<Node<T>>(nodes); } }
}

class Tree<T> where T: class
{
public void DoStuff(Node<T> node, T element)
{
node = new Node<T>();
foreach (var entry in node.Nodes)
if (entry == null)
Debug.Log("Awesome");

Debug.Log("The end");
}
}

public class NewBehaviourScript : MonoBehaviour {
void Start () {
var tree = new Tree<Test>();

tree.DoStuff(null, null);
}
}