Fixes crashes in VM isolate shutdown.

runtime/vm/dart_api_impl.cc

 // Copyright (c) 2013, 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 "include/dart_api.h"
 #include "include/dart_mirrors_api.h"
 #include "include/dart_native_api.h"
 
 #include "platform/assert.h"
 #include "vm/class_finalizer.h"
@@ skipping 5474 matching lines @@
   Dart_Handle state = Api::CheckAndFinalizePendingClasses(isolate);
   if (::Dart_IsError(state)) {
     return state;
   }
 
   // Now that the newly loaded classes are finalized, notify the debugger
   // that new code has been loaded. If there are latent breakpoints in
   // the new code, the debugger convert them to unresolved source breakpoints.
   // The code that completes the futures (invoked below) may call into the
   // newly loaded code and trigger one of these breakpoints.
+  ASSERT(isolate->debugger() != NULL);
   isolate->debugger()->NotifyDoneLoading();
 
   // Notify mirrors that MirrorSystem.libraries needs to be recomputed.
   const Library& libmirrors =
       Library::Handle(isolate, Library::MirrorsLibrary());
   const Field& dirty_bit = Field::Handle(isolate,
       libmirrors.LookupLocalField(String::Handle(String::New("dirty"))));
   ASSERT(!dirty_bit.IsNull() && dirty_bit.is_static());
   dirty_bit.set_value(Bool::True());
 
@@ skipping 101 matching lines @@
 
 
 DART_EXPORT void Dart_RegisterRootServiceRequestCallback(
     const char* name,
     Dart_ServiceRequestCallback callback,
     void* user_data) {
   Service::RegisterRootEmbedderCallback(name, callback, user_data);
 }
 
 }  // namespace dart