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 299 matching lines @@
     } else {
       InvokeEventHandler(&event);
     }
   }
 }
 
 
 void Debugger::SignalIsolateInterrupted() {
   if (HasEventHandler()) {
     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,
                                        Breakpoint* bpt) {
   if (Service::NeedsDebugEvents()) {
@@ skipping 2639 matching lines @@
 }
 
 
 void Debugger::RegisterCodeBreakpoint(CodeBreakpoint* bpt) {
   ASSERT(bpt->next() == NULL);
   bpt->set_next(code_breakpoints_);
   code_breakpoints_ = bpt;
 }
 
 }  // namespace dart