Fix assertion failure in the background compiler.

runtime/vm/object.cc

 // Copyright (c) 2012, 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 "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/become.h"
@@ skipping 5204 matching lines @@
     return false;
   }
   Thread* thread = Thread::Current();
   return thread->isolate()->debugger()->HasBreakpoint(*this, thread->zone());
 }
 
 
 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);
 }
 
 
 void Function::SetInstructions(const Code& value) const {
   DEBUG_ASSERT(IsMutatorOrAtSafepoint());
   SetInstructionsSafe(value);
 }
@@ skipping 17746 matching lines @@
   return UserTag::null();
 }
 
 
 const char* UserTag::ToCString() const {
   const String& tag_label = String::Handle(label());
   return tag_label.ToCString();
 }
 
 }  // namespace dart