Use the right key for the image thumbnail dictionaries in the URL resources struct

[parkera]

The swift importer made a bit of a mess out of the string key here; the real one we want to use is URLThumbnailDictionaryItem.

Fixes rdar://27556178

[parkera]

@swift-ci please test

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - 1553cde
Test requested by - @parkera

[parkera]

@swift-ci please test

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - 1553cde
Test requested by - @parkera

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - 1553cde
Test requested by - @parkera

[jrose-apple]

Just to be clear, we're considering this not a source-breaking change because the old code never worked? It still seems like it could break someone's compilation.

[parkera]

Which "this" are you talking about? Changing the type of the property in the AppKit/UIKit extensions of the URL resources struct or deleting the errant enum?

[jrose-apple]

Both, really. Someone could have been using an explicit type (wrong as it was) and now it won't compile. Maybe leaving behind an unavailable/renamed typealias would help?

[jrose-apple]

(It's still not strictly source-compatible, though.)

[parkera]

I filed another bug for Foundation to figure out the source compat story so I can close out this important fix. 30578582

[parkera]

@swift-ci test and merge

[jrose-apple]

Something funky happened here. I'll restart.

Note that we're past our final branch for Swift 3.1. Should this get cherry-picked there as well?

@swift-ci Please test

[parkera]

We don't need this for Swift 3.1.