Allow using asDexCode for non-DexCode instances

Change-Id: Id901033955846815f5bbefd4a4040fef8791cdfd
diff --git a/src/main/java/com/android/tools/r8/graph/Code.java b/src/main/java/com/android/tools/r8/graph/Code.java
index ee87e8a..e665ee1 100644
--- a/src/main/java/com/android/tools/r8/graph/Code.java
+++ b/src/main/java/com/android/tools/r8/graph/Code.java
@@ -174,7 +174,7 @@
   }
 
   public DexCode asDexCode() {
-    throw new Unreachable(getClass().getCanonicalName() + ".asDexCode()");
+    return null;
   }
 
   public DexWritableCode asDexWritableCode() {