Fixes crashes in VM isolate shutdown.

runtime/vm/isolate.h

 // 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.
 
 #ifndef VM_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/base_isolate.h"
@@ skipping 349 matching lines @@
   static bool IsDeoptBefore(intptr_t deopt_id) {
     return (deopt_id % kDeoptIdStep) == kDeoptIdBeforeOffset;
   }
 
   static bool IsDeoptAfter(intptr_t deopt_id) {
     return (deopt_id % kDeoptIdStep) == kDeoptIdAfterOffset;
   }
 
   Mutex* mutex() const { return mutex_; }
 
-  Debugger* debugger() const { return debugger_; }
+  Debugger* debugger() const {
+    ASSERT(debugger_ != NULL);
+    return debugger_;
+  }
 
   void set_single_step(bool value) { single_step_ = value; }
   bool single_step() const { return single_step_; }
   static intptr_t single_step_offset() {
     return OFFSET_OF(Isolate, single_step_);
   }
 
   void set_has_compiled(bool value) { has_compiled_ = value; }
   bool has_compiled() const { return has_compiled_; }
 
@@ skipping 632 matching lines @@
   intptr_t serialized_args_len_;
   uint8_t* serialized_message_;
   intptr_t serialized_message_len_;
   Isolate::Flags isolate_flags_;
   bool paused_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_