Don't allow OS::SNPrint or OS::VSNPrint to return negative values.

vm/os.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_OS_H_
 #define VM_OS_H_
 
 #include "vm/globals.h"
 
 // Forward declarations.
@@ skipping 44 matching lines @@
   static int NumberOfAvailableProcessors();
 
   // Sleep the currently executing thread for millis ms.
   static void Sleep(int64_t millis);
 
   // Print formatted output to stdout/stderr for debugging.
   static void Print(const char* format, ...);
   static void PrintErr(const char* format, ...);
   static void VFPrint(FILE* stream, const char* format, va_list args);
   // Print formatted output info a buffer.
+  //
   // Does not write more than size characters (including the trailing '\0').
-  // Returns the number of characters (excluding the trailing '\0') that would
-  // been written if the buffer had been big enough.
-  // If the return value is greater or equal than the given size then the output
-  // has been truncated.
-  // The buffer will always be terminated by a '\0', unless the buffer is of
-  // size 0.
-  // The buffer might be NULL if the size is 0.
-  // This specification conforms to C99 standard which is implemented by
-  // glibc 2.1+.
+  //
+  // Returns the number of characters (excluding the trailing '\0')
+  // that would been written if the buffer had been big enough.  If
+  // the return value is greater or equal than the given size then the
+  // output has been truncated.  The return value is never negative.
+  //
+  // The buffer will always be terminated by a '\0', unless the buffer
+  // is of size 0.  The buffer might be NULL if the size is 0.
+  //
+  // This specification conforms to C99 standard which is implemented
+  // by glibc 2.1+ with one exception: the C99 standard allows a
+  // negative return value.  We will terminate the vm rather than let
+  // that occur.
   static int SNPrint(char* str, size_t size, const char* format, ...);
   static int VSNPrint(char* str, size_t size,
                       const char* format,
                       va_list args);
 
   // Initialize the OS class.
   static void InitOnce();
 
   // Shut down the OS class.
   static void Shutdown();
 
   static void Abort();
 
   static void Exit(int code);
 };
 
 }  // namespace dart
 
 #endif  // VM_OS_H_