Bug 1469712 - Don't update mtime for backend file during --dry-run; r=gps

When we do a './mach build-backend --dry-run', no files in the objdir
should be written or touched. Updating the mtime of the backend file
during dry-run causes a future './mach build' to think that the backend
is up-to-date when it is not, which causes an incorrect build.

MozReview-Commit-ID: 2NA0rcSGrvL

--HG--
extra : amend_source : f694503264dc6275568c453de59b7f13983091f7

python/mozbuild/mozbuild/backend/base.py

@@ -174,9 +174,10 @@ class BuildBackend(LoggingMixin):
             with self._write_file(list_file) as fh:
                 fh.write('\n'.join(sorted(self._backend_output_files)))
         else:
-            # Always update its mtime.
-            with open(list_file, 'a'):
-                os.utime(list_file, None)
+            # Always update its mtime if we're not in dry-run mode.
+            if not self.dry_run:
+                with open(list_file, 'a'):
+                    os.utime(list_file, None)
 
         # Write out the list of input files for the backend
         with self._write_file('%s.in' % list_file) as fh: