Build failed. Method not found?

Build failed. Method not found: ‘Int32 System.Environment.get_CurrentManagedThreadId()’.
Build: 1 error, 0 warnings’

What does this mean?

It is the error I get with this script:

using UnityEngine;
using System.Collections;

public class Test : MonoBehaviour {

// Use this for initialization
	void Start () {
	float speed = 10;
}

// Update is called once per frame
void Update () {
	if (KeyCode.UpArrow){
		//transform.position += Vector3.up * Time.deltaTime; }
		transform.Translate(Vector3.right * speed * Time.deltaTime);
	}
}

Does this thread help?

http://forum.unity3d.com/threads/monodevelop-build-failed.275506/

or this?