Tearing off a top-level variable introduces a synthetic field which is added to the top-level class…

runtime/vm/precompiler.cc

 // Copyright (c) 2015, 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/precompiler.h"
 
 #include "vm/aot_optimizer.h"
 #include "vm/assembler.h"
 #include "vm/ast_printer.h"
 #include "vm/branch_optimizer.h"
@@ skipping 1703 matching lines @@
         field ^= fields.At(j);
         bool retain = fields_to_retain_.Lookup(&field) != NULL;
         if (retain) {
           retained_fields.Add(field);
           type = field.type();
           AddType(type);
         } else {
           bool top_level = cls.IsTopLevel();
           if (top_level) {
             name = field.DictionaryName();
-            bool removed = lib.RemoveObject(field, name);
-            ASSERT(removed);
+            lib.RemoveObject(field, name);
           }
           dropped_field_count_++;
           if (FLAG_trace_precompiler) {
             THR_Print("Dropping field %s\n",
                       field.ToCString());
           }
         }
       }
 
       if (retained_fields.Length() > 0) {
@@ skipping 1604 matching lines @@
 
   ASSERT(FLAG_precompiled_mode);
   const bool optimized = function.IsOptimizable();  // False for natives.
   DartPrecompilationPipeline pipeline(zone, field_type_map);
   return PrecompileFunctionHelper(&pipeline, function, optimized);
 }
 
 #endif  // DART_PRECOMPILER
 
 }  // namespace dart