[vm] Fix MSVC build.

TEST=vm-win-release-arm64
Change-Id: I6aff6fc17441d366885b0da59c519343832beb67
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/365922
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
diff --git a/runtime/vm/flags.cc b/runtime/vm/flags.cc
index 7ccc6ed..7b125b9 100644
--- a/runtime/vm/flags.cc
+++ b/runtime/vm/flags.cc
@@ -73,21 +73,23 @@
 #undef DEBUG_FLAG_MACRO
 
 #if defined(DART_PRECOMPILER)
+#if defined(TARGET_USES_THREAD_SANITIZER)
+constexpr bool kDefaultTargetThreadSanitizer = true;
+#else
+constexpr bool kDefaultTargetThreadSanitizer = false;
+#endif
 DEFINE_FLAG(bool,
             target_thread_sanitizer,
-#if defined(TARGET_USES_THREAD_SANITIZER)
-            true,
-#else
-            false,
-#endif
+            kDefaultTargetThreadSanitizer,
             "Generate Dart code compatible with Thread Sanitizer");
+#if defined(TARGET_USES_MEMORY_SANITIZER)
+constexpr bool kDefaultTargetMemorySanitizer = true;
+#else
+constexpr bool kDefaultTargetMemorySanitizer = false;
+#endif
 DEFINE_FLAG(bool,
             target_memory_sanitizer,
-#if defined(TARGET_USES_MEMORY_SANITIZER)
-            true,
-#else
-            false,
-#endif
+            kDefaultTargetMemorySanitizer,
             "Generate Dart code compatible with Memory Sanitizer");
 #endif