Prefix compile-time typedef checks with __attribute__((unused))

runtime/platform/assert.h

Index: runtime/platform/assert.h
diff --git a/runtime/platform/assert.h b/runtime/platform/assert.h
index 3b5f32e6e4609f460bf9dd980b26a9482fea1997..73193baa3c1112b6209ffa6843221b048c1954a0 100644
--- a/runtime/platform/assert.h
+++ b/runtime/platform/assert.h
@@ -289,8 +289,8 @@ template <bool>
 struct CompileAssert {
 };
 
-#define COMPILE_ASSERT(expr, msg)                       \
-  typedef CompileAssert<(static_cast<bool>(expr))>      \
+#define COMPILE_ASSERT(expr, msg)                                              \
+  DART_UNUSED typedef CompileAssert<(static_cast<bool>(expr))>                 \
   msg[static_cast<bool>(expr) ? 1 : -1]