[modules] Make duplicate export error deterministic.

test/message/export-duplicate.out

Index: test/message/export-duplicate.out
diff --git a/test/message/export-duplicate.out b/test/message/export-duplicate.out
index e88779f5805370e47565c87126674e4b934d7c19..9811cb122cd0ef3f98f595611fa32f5d510aad24 100644
--- a/test/message/export-duplicate.out
+++ b/test/message/export-duplicate.out
@@ -1,7 +1,7 @@
 # Copyright 2015 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
-*%(basename)s:9: SyntaxError: Duplicate export of 'a'
-export { a, b };
+*%(basename)s:10: SyntaxError: Duplicate export of 'b'
+export { b, c };
          ^
-SyntaxError: Duplicate export of 'a'
+SyntaxError: Duplicate export of 'b'