And then I found this:
Objective-C expression evaluation implicitly imports all modules that were imported in the source context where execution takes place, providing a much higher fidelity debugging experience. In most cases this change eliminates the need to manually import modules while debugging. For example, using expr — @import UIKit that was commonly used to ensure that SDK declarations were available is no longer necessary. (11609847)
I had already imported UIKit into a playground module, so I didn’t need to re-import it into my Xcode 7.3/Swift 2.2 main playground page.