Use -Xno-dex-file-fallback for libcore tests.

Change-Id: I23911d17c4d71504b3ff9293325af533097ba49d
diff --git a/tools/run-libcore-tests.sh b/tools/run-libcore-tests.sh
index 1b8748b..394981bd 100755
--- a/tools/run-libcore-tests.sh
+++ b/tools/run-libcore-tests.sh
@@ -64,6 +64,8 @@
                   "jsr166")
 
 vogar_args=$@
+# Make sure we don't fall back to dex files without oat files.
+vogar_args="$vogar_args --vm-arg -Xno-dex-file-fallback"
 while true; do
   if [[ "$1" == "--mode=device" ]]; then
     vogar_args="$vogar_args --device-dir=/data/local/tmp"