XFAIL 3 tests on darwin 11-12.  The tests have recently been modified, are correct, and pass with an updated libc++.dylib

llvm-svn: 188964
diff --git a/libcxx/test/strings/string.conversions/stoull.pass.cpp b/libcxx/test/strings/string.conversions/stoull.pass.cpp
index 19df706..b1a2f9f 100644
--- a/libcxx/test/strings/string.conversions/stoull.pass.cpp
+++ b/libcxx/test/strings/string.conversions/stoull.pass.cpp
@@ -6,6 +6,9 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+//
+// XFAIL: with_system_lib=x86_64-apple-darwin11
+// XFAIL: with_system_lib=x86_64-apple-darwin12
 
 // <string>