Emit information about computation on assertion error

Change-Id: Id9bfe81cd7e70da0e21e0b336b5d4092de227dde
diff --git a/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/InFlowComparator.java b/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/InFlowComparator.java
index 1327a44..fd7a92d 100644
--- a/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/InFlowComparator.java
+++ b/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/InFlowComparator.java
@@ -23,7 +23,8 @@
 
   public SourcePosition getComputationTreePosition(ComputationTreeNode computation) {
     SourcePosition position = computationTreePositions.get(computation);
-    assert position != null;
+    assert position != null
+        : "Unexpected attempt to lookup position of " + computation.getClass().getName();
     return position;
   }