Handle synthesizing class contexts in missing classes

Bug: 183734568
Change-Id: I659a4aed6edb24dea31cf5b0cbdc5aef8d6939b1
diff --git a/src/main/java/com/android/tools/r8/shaking/MissingClasses.java b/src/main/java/com/android/tools/r8/shaking/MissingClasses.java
index 317a4cd..0330dca 100644
--- a/src/main/java/com/android/tools/r8/shaking/MissingClasses.java
+++ b/src/main/java/com/android/tools/r8/shaking/MissingClasses.java
@@ -204,6 +204,10 @@
                 holder.lookupProgramMethod(synthesizingContextReference.asDexMethod());
             assert synthesizingContext != null;
             rewrittenContexts.add(synthesizingContext);
+          } else if (synthesizingContextReference.isDexType()) {
+            DexProgramClass synthesizingClass =
+                appView.definitionFor(synthesizingContextReference.asDexType()).asProgramClass();
+            rewrittenContexts.add(synthesizingClass);
           } else {
             assert false
                 : "Unexpected synthesizing context: "