title: Horizontal Constraint causes crash in iOS
body: <p>I have two views. <code>subLabel</code> and <code>tinyImage</code></p>
  
  <p>I'd like the tinyImage to be right of subLabel, with a 10 pixel spacing.  This is what I have:</p>
  
  <pre><code>        let horizontalConstraint = NSLayoutConstraint(item: subLabel, attribute: NSLayoutAttribute.Trailing, relatedBy: NSLayoutRelation.Equal, toItem: tinyImage, attribute: NSLayoutAttribute.Leading, multiplier: 1, constant: 10)
          tinyImage.addConstraint(horizontalConstraint)
  </code></pre>
  
  <p>When I run this code, it crashes, and I see something like:</p>
  
  <pre><code>The view hierarchy is not prepared for the constraint: &lt;NSLayoutConstraint:0x7fe89a6ece10 UILabel:0x7fe89a764cf0'2'.trailing == UIImageView:0x7fe89a765bb0.leading + 10&gt;
      When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:] to debug.
  </code></pre>