Fix assertion failure in the background compiler.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 2da9eb62c4b0b5c7f5ef255576b59967b0fba013..5c0663fb65335bf02efe2f1c9ecbca0c2a2144e1 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -5222,7 +5222,8 @@ bool Function::HasBreakpoint() const {
 void Function::InstallOptimizedCode(const Code& code, bool is_osr) const {
   DEBUG_ASSERT(IsMutatorOrAtSafepoint());
   // We may not have previous code if FLAG_precompile is set.
-  if (!is_osr && HasCode()) {
+  // Hot-reload may have already disabled the current code.
+  if (!is_osr && HasCode() && !Code::Handle(CurrentCode()).IsDisabled()) {
     Code::Handle(CurrentCode()).DisableDartCode();
   }
   AttachCode(code);