Fix duplication of the megamorphic miss function when running a JIT app snapshot.

runtime/vm/megamorphic_cache_table.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/megamorphic_cache_table.h"
 
 #include <stdlib.h>
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/stub_code.h"
@@ skipping 58 matching lines @@
                     false,  // Not external.
                     false,  // Not native.
                     cls, TokenPosition::kNoSource));
   function.set_result_type(Type::Handle(Type::DynamicType()));
   function.set_is_debuggable(false);
   function.set_is_visible(false);
   function.AttachCode(code);  // Has a single entry point, as a static function.
   // For inclusion in Snapshot::kAppJIT.
   function.set_unoptimized_code(code);
 
+  ASSERT(isolate->object_store()->megamorphic_miss_function() ==
+         Function::null());
   isolate->object_store()->SetMegamorphicMissHandler(code, function);
 }
 
 
 void MegamorphicCacheTable::PrintSizes(Isolate* isolate) {
   StackZone zone(Thread::Current());
   intptr_t size = 0;
   MegamorphicCache& cache = MegamorphicCache::Handle();
   Array& buckets = Array::Handle();
   const GrowableObjectArray& table = GrowableObjectArray::Handle(
@@ skipping 52 matching lines @@
   for (intptr_t i = 0; i <= max_probe_count; i++) {
     cumulative_entries += probe_counts[i];
     OS::Print("Megamorphic probe %" Pd ": %" Pd " (%lf)\n", i, probe_counts[i],
               static_cast<double>(cumulative_entries) /
                   static_cast<double>(entry_count));
   }
   delete[] probe_counts;
 }
 
 }  // namespace dart