Move fallback calculations to later in the transition lifecycle.

Summary: This ensures that fallback calculations have access to the containerView property.

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D3239

src/transitions/TransitionContext.swift

@@ -111,14 +111,6 @@ public final class TransitionContext: NSObject {
     self.transition = transition
 
     super.init()
-
-    while let fallbackTransition = self.transition as? TransitionWithFallback {
-      let fallback = fallbackTransition.fallbackTansition(withContext: self)
-      if fallback === self.transition {
-        break
-      }
-      self.transition = fallback
-    }
   }
 
   fileprivate let initialDirection: TransitionDirection
@@ -182,6 +174,16 @@ extension TransitionContext {
     self.runtime = MotionRuntime(containerView: containerView())
     self.replicator.containerView = containerView()
 
+    // We query the fallback just before initiating the transition so that the transition context is
+    // primed with the content view and other transition-related information.
+    while let fallbackTransition = self.transition as? TransitionWithFallback {
+      let fallback = fallbackTransition.fallbackTansition(withContext: self)
+      if fallback === self.transition {
+        break
+      }
+      self.transition = fallback
+    }
+
     pokeSystemAnimations()
 
     var terminators = transition.willBeginTransition(withContext: self, runtime: self.runtime)