Don't require r8lib.map to be present.

Change-Id: I0c9897e68f1f5b54daaa69fa7f704a79f5d5f35b
diff --git a/tools/retrace.py b/tools/retrace.py
index a7ff23d..cbb2d33 100755
--- a/tools/retrace.py
+++ b/tools/retrace.py
@@ -3,6 +3,8 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
+from os import path
+
 import argparse
 import os
 import subprocess
@@ -122,7 +124,7 @@
     else:
       args.commit_hash = r8_version_or_hash
     map_path = None
-    if get_hash_from_map_file(utils.R8LIB_MAP) == maphash:
+    if path.exists(utils.R8LIB_MAP) and get_hash_from_map_file(utils.R8LIB_MAP) == maphash:
       return utils.R8LIB_MAP
 
     try: