Fixes crashes in VM isolate shutdown.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 2432df990bfa34e8945bc702086c827c4bef13c6..929f46064a822bc753e6f1a0f46789aedafcfb9c 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -518,6 +518,7 @@ bool IsolateMessageHandler::ProcessUnhandledException(const Error& result) {
     if ((exception == I->object_store()->out_of_memory()) ||
         (exception == I->object_store()->stack_overflow())) {
       // We didn't notify the debugger when the stack was full. Do it now.
+      ASSERT(I->debugger() != NULL);
       I->debugger()->SignalExceptionThrown(Instance::Handle(exception));
     }
   }
@@ -775,8 +776,10 @@ Isolate* Isolate::Init(const char* name_prefix,
   result->set_terminate_capability(result->random()->NextUInt64());
 
   result->BuildName(name_prefix);
-  result->debugger_ = new Debugger();
-  result->debugger_->Initialize(result);
+  if (!is_vm_isolate) {
+    result->debugger_ = new Debugger();
+    result->debugger_->Initialize(result);
+  }
   if (FLAG_trace_isolates) {
     if (name_prefix == NULL || strcmp(name_prefix, "vm-isolate") != 0) {
       OS::Print("[+] Starting isolate:\n"
@@ -1221,6 +1224,7 @@ static bool RunIsolate(uword parameter) {
     // of its implicit closure function because that latter is merely
     // a dispatcher that is marked as undebuggable.
     if (FLAG_break_at_isolate_spawn) {
+      ASSERT(isolate->debugger() != NULL);
       isolate->debugger()->OneTimeBreakAtEntry(func);
     }
 
@@ -1425,7 +1429,9 @@ void Isolate::Shutdown() {
     }
 
     // Clean up debugger resources.
-    debugger()->Shutdown();
+    if (debugger() != NULL) {
+      debugger()->Shutdown();
+    }
 
     // Close all the ports owned by this isolate.
     PortMap::ClosePorts(message_handler());
@@ -1526,6 +1532,7 @@ void Isolate::VisitObjectPointers(ObjectPointerVisitor* visitor,
       reinterpret_cast<RawObject**>(&deoptimized_code_array_));
 
   // Visit objects in the debugger.
+  ASSERT(debugger() != NULL);

[Ivan Posva, 2015/06/16 16:31:15]

if (debugger() != NULL) {
  ...
}

Since we are visiting the VM isolate in certain situations.

   debugger()->VisitObjectPointers(visitor);
 
   // Visit objects that are being used for deoptimization.
@@ -1582,6 +1589,7 @@ void Isolate::PrintJSON(JSONStream* stream, bool ref) {
   jsobj.AddProperty("livePorts", message_handler()->live_ports());
   jsobj.AddProperty("pauseOnExit", message_handler()->pause_on_exit());
 
+  ASSERT(debugger() != NULL);

[Ivan Posva, 2015/06/16 16:31:15]

We should allow printing of the VM isolate.

   if (message_handler()->paused_on_start()) {
     ASSERT(debugger()->PauseEvent() == NULL);
     ServiceEvent pause_event(this, ServiceEvent::kPauseStart);