Fix following compiler warnings:

runtime/vm/os_win.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "vm/os.h"
 
 #include <malloc.h>  // NOLINT
@@ skipping 245 matching lines @@
     va_list args_retry;
     va_copy(args_retry, args);
     written = _vscprintf(format, args_retry);
     if (written < 0) {
       FATAL1("Fatal error in OS::VSNPrint with format '%s'", format);
     }
     va_end(args_retry);
   }
   // Make sure to zero-terminate the string if the output was
   // truncated or if there was an error.
-  if (written >= size) {
+  // The static cast is safe here as we have already determined that 'written'
+  // is >= 0.
+  if (static_cast<size_t>(written) >= size) {
     str[size - 1] = '\0';
   }
   return written;
 }
 
 
 bool OS::StringToInt64(const char* str, int64_t* value) {
   ASSERT(str != NULL && strlen(str) > 0 && value != NULL);
   int32_t base = 10;
   char* endptr;
@@ skipping 50 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)