[Unity v5.3] Comes with an Editor that allows easily working around the Polygon2DCollider bug. WARNING: Make sure to place the Editor script in an Editor folder!

ImprovedPolygonCollider2D.cs

/**
 * Comes with an Editor that allows easily working around the Polygon2DCollider bug.
 */

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(PolygonCollider2D))]
public class ImprovedPolygonCollider2D : MonoBehaviour {
}

ImprovedPolygonCollider2DEditor.cs

/**
 * Provides a simple Reset button to quickly fix a broken Polygon2DCollider.
 * WARNING: Make sure to place this script in an "Editor" folder.
 */

using UnityEngine;
using UnityEditor;

using System.Linq;

[CustomEditor(typeof(ImprovedPolygonCollider2D))]
public class ImprovedPolygonCollider2DEditor : Editor {
    public override void OnInspectorGUI() {
        base.OnInspectorGUI();

        var theTarget = (ImprovedPolygonCollider2D)target;
        var spriteRenderer = theTarget.GetComponent<SpriteRenderer>();
        var collider = theTarget.GetComponent<PolygonCollider2D>();

        if (spriteRenderer != null && collider != null && GUILayout.Button("Reset")) {
            ReplaceWithSimpleBox();
        }
    }

    void ReplaceWithSimpleBox() {
        var theTarget = (ImprovedPolygonCollider2D)target;
        var spriteRenderer = theTarget.GetComponent<SpriteRenderer>();
        var collider = theTarget.GetComponent<PolygonCollider2D>();

        // get bounds
        var bounds = spriteRenderer.bounds;
        Vector2 p1 = bounds.min;
        Vector2 p3 = bounds.max;
        Vector2 p2 = new Vector2(p1.x, p3.y);
        Vector2 p4 = new Vector2(p3.x, p1.y);
        collider.points = new Vector2[] { p1, p2, p3, p4 };

        // convert to object space
        collider.points = collider.points.Select(p => (Vector2)collider.transform.InverseTransformPoint(p)).ToArray();

        EditorUtility.SetDirty(target);
    }
}