Fixes crashes in VM isolate shutdown.

runtime/vm/debugger.cc

Index: runtime/vm/debugger.cc
diff --git a/runtime/vm/debugger.cc b/runtime/vm/debugger.cc
index 939dafdd8fdfab7866e6af741d3633fe4051c6e6..80c12cea7f5023b99756ce3a29c26dafb4a915af 100644
--- a/runtime/vm/debugger.cc
+++ b/runtime/vm/debugger.cc
@@ -316,6 +316,7 @@ void Debugger::SignalIsolateEvent(DebuggerEvent::EventType type) {
 
 void Debugger::SignalIsolateInterrupted() {
   if (HasEventHandler()) {
+    ASSERT(Isolate::Current()->debugger() != NULL);
     Debugger* debugger = Isolate::Current()->debugger();
     ASSERT(debugger != NULL);
     debugger->SignalIsolateEvent(DebuggerEvent::kIsolateInterrupted);
@@ -844,6 +845,7 @@ void ActivationFrame::PrintContextMismatchError(
 
   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();