blueView.swift

func setupBlueViewConstraints() {

  blueView.translatesAutoresizingMaskIntoConstraints = false

  let blueViewTopSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .Top, relatedBy: .Equal, toItem:view, attribute: .Top, multiplier: 1.0, constant: 8)
  let blueViewBottomSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .Bottom, relatedBy: .Equal, toItem: orangeView, attribute: .Top, multiplier: 1.0, constant: -8.0)
  let blueViewTrailingSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .TrailingMargin, multiplier: 1.0, constant: 8.0)
  let equalWidthConstraints = NSLayoutConstraint(item: blueView, attribute: .Width, relatedBy: .Equal, toItem: purpleView, attribute: .Width, multiplier: 1.0, constant: 0.0)
  
  view.addConstraints([blueViewTopSpaceConstraint, blueViewBottomSpaceConstraint, blueViewTrailingSpaceConstraint, equalWidthConstraints])
}

There's a typo for blueViewTrailingSpaceConstraint:
The constant should be -8.0, not 8.0.

Incorrect:

let blueViewTrailingSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .TrailingMargin, multiplier: 1.0, constant: 8.0)

Correct:

let blueViewTrailingSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .Trailing, relatedBy: .Equal, toItem: view, attribute: .TrailingMargin, multiplier: 1.0, constant: -8.0)

Thanks @davejlin

It's a trivial change for Swift 3 and shouldn't cause any trouble for students to go through and fix, but Xcode complains loudly about the enum casing. The Swift 3 equivalent is:

func setupBlueViewConstraints() {
        
        blueView.translatesAutoresizingMaskIntoConstraints = false
        
        let blueViewTopSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .top, relatedBy: .equal, toItem:view, attribute: .top, multiplier: 1.0, constant: 8)
        let blueViewBottomSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .bottom, relatedBy: .equal, toItem: orangeView, attribute: .top, multiplier: 1.0, constant: -8.0)
        let blueViewTrailingSpaceConstraint = NSLayoutConstraint(item: blueView, attribute: .trailing, relatedBy: .equal, toItem: view, attribute: .trailingMargin, multiplier: 1.0, constant: -8.0)
        let equalWidthConstraints = NSLayoutConstraint(item: blueView, attribute: .width, relatedBy: .equal, toItem: purpleView, attribute: .width, multiplier: 1.0, constant: 0.0)
        
        view.addConstraints([blueViewTopSpaceConstraint, blueViewBottomSpaceConstraint, blueViewTrailingSpaceConstraint, equalWidthConstraints])
    }

Thanks @NFulkerson