Fixes a nasty issue in gtest's template instantiation.

include/gtest/internal/gtest-port.h

@@ -32,6 +32,10 @@
 // Low-level types and utilities for porting Google Test to various
 // platforms.  They are subject to change without notice.  DO NOT USE
 // THEM IN USER CODE.
+//
+// This file is fundamental to Google Test.  All other Google Test source
+// files are expected to #include this.  Therefore, it cannot #include
+// any other Google Test header.
 
 #ifndef GTEST_INCLUDE_GTEST_INTERNAL_GTEST_PORT_H_
 #define GTEST_INCLUDE_GTEST_INTERNAL_GTEST_PORT_H_
@@ -784,6 +788,11 @@ class Message;
 
 namespace internal {
 
+// A secret type that Google Test users don't know about.  It has no
+// definition on purpose.  Therefore it's impossible to create a
+// Secret object, which is what we want.
+class Secret;
+
 // The GTEST_COMPILE_ASSERT_ macro can be used to verify that a compile time
 // expression is true. For example, you could use it to verify the
 // size of a static array: