Build and archive map files for R8-processed targets.

Change-Id: Id673cd21d82b208a6e1038ae03569d9ea8af5457
diff --git a/tools/build_r8lib.py b/tools/build_r8lib.py
index 5af6d4d..5368f6f 100755
--- a/tools/build_r8lib.py
+++ b/tools/build_r8lib.py
@@ -56,7 +56,7 @@
   # Produce R8 for compiling lib
   if output_path is None:
     output_path = target + 'lib.jar'
-  output_map_path = os.path.splitext(output_path)[0] + '.map'
+  output_map_path = output_path + '.map'
   toolhelper.run(
       'r8',
       ('--release',