Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/thread.h

 // Copyright (c) 2015, 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 RUNTIME_VM_THREAD_H_
 #define RUNTIME_VM_THREAD_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 789 matching lines @@
   friend class IsolateTestHelper;
   friend class NoOOBMessageScope;
   friend class Simulator;
   friend class StackZone;
   friend class ThreadRegistry;
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 
-#if defined(TARGET_OS_WINDOWS)
+#if defined(HOST_OS_WINDOWS)
 // Clears the state of the current thread and frees the allocation.
 void WindowsThreadCleanUp();
 #endif
 
 
 // Disable thread interrupts.
 class DisableThreadInterruptsScope : public StackResource {
  public:
   explicit DisableThreadInterruptsScope(Thread* thread);
   ~DisableThreadInterruptsScope();
 };
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_THREAD_H_