Merge pull request #3691 from pherl/stringback

Fix C++11 string accessors

src/google/protobuf/testing/file.cc

diff --git a/src/google/protobuf/testing/file.cc b/src/google/protobuf/testing/file.cc
index f32222b..f68aba9 100644
--- a/src/google/protobuf/testing/file.cc
+++ b/src/google/protobuf/testing/file.cc
@@ -121,7 +121,7 @@
 
 bool File::CreateDir(const string& name, int mode) {
   if (!name.empty()) {
-    GOOGLE_CHECK_OK(name.back() != '.');
+    GOOGLE_CHECK_OK(name[name.size() - 1] != '.');
   }
   return mkdir(name.c_str(), mode) == 0;
 }