Add context to assertion failure

Bug: b/401494301
Change-Id: I514d5f0ac1bb51bc3d68ffd72907880cbb7f3d6c

src/main/java/com/android/tools/r8/ir/code/IRCode.java

diff --git a/src/main/java/com/android/tools/r8/ir/code/IRCode.java b/src/main/java/com/android/tools/r8/ir/code/IRCode.java
index 133cc8d..50e4f53 100644
--- a/src/main/java/com/android/tools/r8/ir/code/IRCode.java
+++ b/src/main/java/com/android/tools/r8/ir/code/IRCode.java
@@ -670,7 +670,11 @@
             .anyMatch(
                 (itf, isKnown) -> {
                   assert !mergedClasses.isMergeSource(itf)
-                      : "Expected " + itf + " to not be a merge source";
+                      : "Expected "
+                          + itf
+                          + " to not be a merge source (context is "
+                          + method.toSourceString()
+                          + ")";
                   return false;
                 });
       }