Don't write Code::code_source_map_ in snapshots.

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 224 matching lines @@
     DropFunctions();
     DropFields();
     TraceTypesFromRetainedClasses();
     DropTypes();
     DropTypeArguments();
 
     // Clear these before dropping classes as they may hold onto otherwise
     // dead instances of classes we will remove.
     I->object_store()->set_compile_time_constants(Array::null_array());
     I->object_store()->set_unique_dynamic_targets(Array::null_array());
+    Class& null_class = Class::Handle(Z);
+    I->object_store()->set_future_class(null_class);
+    I->object_store()->set_completer_class(null_class);
+    I->object_store()->set_stream_iterator_class(null_class);
+    I->object_store()->set_symbol_class(null_class);
 
     DropClasses();
     DropLibraries();
 
     BindStaticCalls();
     SwitchICCalls();
 
     DedupStackmaps();
     DedupStackmapLists();
 
@@ skipping 2583 matching lines @@
       CompilationPipeline::New(thread->zone(), function);
 
   ASSERT(FLAG_precompiled_mode);
   const bool optimized = function.IsOptimizable();  // False for natives.
   return PrecompileFunctionHelper(pipeline, function, optimized);
 }
 
 #endif  // DART_PRECOMPILER
 
 }  // namespace dart