Allow missing classes in R8 mk builds

Bug: 180878971
Bug: 72683872
Test: TARGET_PRODUCT=aosp_arm64 m -j core-oj
Change-Id: I4e3d49d4d8aa7e373bae6710db57f503eb95fab1
diff --git a/core/definitions.mk b/core/definitions.mk
index de95890..2883f0d 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -2787,7 +2787,8 @@
     $(R8_DEBUG_MODE) \
     $(PRIVATE_PROGUARD_FLAGS) \
     $(addprefix -injars , $(PRIVATE_EXTRA_INPUT_JAR)) \
-    $(PRIVATE_DX_FLAGS)
+    $(PRIVATE_DX_FLAGS) \
+    -ignorewarnings
 $(hide) touch $(PRIVATE_PROGUARD_DICTIONARY)
 endef