Rename TARGET_OS_* to HOST_OS_*.

runtime/vm/malloc_hooks_test.cc

 // Copyright (c) 2017, 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 "platform/globals.h"
 
 #if defined(DART_USE_TCMALLOC) && !defined(PRODUCT) &&                         \
-    !defined(TARGET_ARCH_DBC) && !defined(TARGET_OS_FUCHSIA)
+    !defined(TARGET_ARCH_DBC) && !defined(HOST_OS_FUCHSIA)
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/malloc_hooks.h"
 #include "vm/os.h"
 #include "vm/profiler.h"
 #include "vm/profiler_service.h"
 #include "vm/unit_test.h"
 
 namespace dart {
@@ skipping 181 matching lines @@
 
   free(var);
   MallocHooks::TearDown();
   MallocHooks::set_stack_trace_collection_enabled(enable_stack_traces_saved);
   FLAG_enable_malloc_hooks = enable_malloc_hooks_saved;
 }
 
 };  // namespace dart
 
 #endif  // defined(DART_USE_TCMALLOC) && !defined(PRODUCT)