Fixes crashes in VM isolate shutdown.

runtime/vm/debugger.cc

 // Copyright (c) 2012, 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/debugger.h"
 
 #include "include/dart_api.h"
 
 #include "vm/code_generator.h"
 #include "vm/code_patcher.h"
@@ skipping 298 matching lines @@
       stack_trace_ = NULL;
     } else {
       InvokeEventHandler(&event);
     }
   }
 }
 
 
 void Debugger::SignalIsolateInterrupted() {
   if (HasEventHandler()) {
+    ASSERT(Isolate::Current()->debugger() != NULL);
     Debugger* debugger = Isolate::Current()->debugger();
     ASSERT(debugger != NULL);
     debugger->SignalIsolateEvent(DebuggerEvent::kIsolateInterrupted);
   }
 }
 
 
 // The vm service handles breakpoint notifications in a different way
 // than the regular debugger breakpoint notifications.
 static void SendServiceBreakpointEvent(ServiceEvent::EventType type,
@@ skipping 508 matching lines @@
                "Current frame:\n%s\n",
                this->ToCString());
 
   OS::PrintErr("-------------------------\n"
                "Context contents:\n");
   const Context& ctx = GetSavedCurrentContext();
   ctx.Dump(8);
 
   OS::PrintErr("-------------------------\n"
                "Debugger stack trace...\n\n");
+  ASSERT(Isolate::Current()->debugger() != NULL);
   DebuggerStackTrace* stack =
       Isolate::Current()->debugger()->StackTrace();
   intptr_t num_frames = stack->Length();
   for (intptr_t i = 0; i < num_frames; i++) {
     ActivationFrame* frame = stack->FrameAt(i);
     OS::PrintErr("#%04" Pd " %s", i, frame->ToCString());
   }
 
   OS::PrintErr("-------------------------\n"
                "All frames...\n\n");
@@ skipping 2095 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart