Add missing return statement to fix Windows builds

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 114 matching lines @@
 
 
 int64_t OS::GetCurrentMonotonicTicks() {
   if (qpc_ticks_per_second == 0) {
     // QueryPerformanceCounter not supported, fallback.
     return GetCurrentTimeMicros();
   }
   // Grab performance counter value.
   LARGE_INTEGER now;
   QueryPerformanceCounter(&now);
-  int64_t qpc_value = static_cast<int64_t>(now.QuadPart);
+  return static_cast<int64_t>(now.QuadPart);
 }
 
 
 int64_t OS::GetCurrentMonotonicFrequency() {
   if (qpc_ticks_per_second == 0) {
     // QueryPerformanceCounter not supported, fallback.
     return kMicrosecondsPerSecond;
   }
   return qpc_ticks_per_second;
 }
@@ skipping 262 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)