Fixes crashes in VM isolate shutdown.

runtime/vm/dart_entry.cc

 // Copyright (c) 2011, 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/dart_entry.h"
 
 #include "vm/class_finalizer.h"
 #include "vm/code_generator.h"
 #include "vm/compiler.h"
 #include "vm/debugger.h"
@@ skipping 473 matching lines @@
                                        class_name,
                                        function_name,
                                        kNumArguments,
                                        Object::empty_array());
     ASSERT(!function.IsNull());
     isolate->object_store()->set_handle_message_function(function);
   }
   const Array& args = Array::Handle(isolate, Array::New(kNumArguments));
   args.SetAt(0, handler);
   args.SetAt(1, message);
+  ASSERT(isolate->debugger() != NULL);
   if (isolate->debugger()->IsStepping()) {
     // If the isolate is being debugged and the debugger was stepping
     // through code, enable single stepping so debugger will stop
     // at the first location the user is interested in.
     isolate->debugger()->SetSingleStep();
   }
   const Object& result = Object::Handle(isolate,
       DartEntry::InvokeFunction(function, args));
   ASSERT(result.IsNull() || result.IsError());
   return result.raw();
@@ skipping 14 matching lines @@
   const Array& args = Array::Handle(Array::New(kNumArguments));
   args.SetAt(0, map);
   args.SetAt(1, key);
   args.SetAt(2, value);
   const Object& result = Object::Handle(DartEntry::InvokeFunction(function,
                                                                   args));
   return result.raw();
 }
 
 }  // namespace dart