Allow missing classes in R8 builds

Bug: 180878971
Bug: 72683872
Test: TARGET_PRODUCT=aosp_arm64 m -j core-oj
Change-Id: I663220567eb8f034d3efccc60228aef91c716e8e
diff --git a/java/dex.go b/java/dex.go
index e52fdb5..b2a998f 100644
--- a/java/dex.go
+++ b/java/dex.go
@@ -260,6 +260,9 @@
 		r8Flags = append(r8Flags, "--debug")
 	}
 
+	// TODO(b/180878971): missing classes should be added to the relevant builds.
+	r8Flags = append(r8Flags, "-ignorewarnings")
+
 	return r8Flags, r8Deps
 }