Fixes crashes in VM isolate shutdown.

runtime/vm/exceptions.cc

 // Copyright (c) 2011, 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/exceptions.h"
 
 #include "platform/address_sanitizer.h"
 
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
@@ skipping 464 matching lines @@
 }
 
 
 void Exceptions::Throw(Thread* thread, const Instance& exception) {
   // Do not notify debugger on stack overflow and out of memory exceptions.
   // The VM would crash when the debugger calls back into the VM to
   // get values of variables.
   Isolate* isolate = thread->isolate();
   if (exception.raw() != isolate->object_store()->out_of_memory() &&
       exception.raw() != isolate->object_store()->stack_overflow()) {
+    ASSERT(isolate->debugger() != NULL);
     isolate->debugger()->SignalExceptionThrown(exception);
   }
   // Null object is a valid exception object.
   ThrowExceptionHelper(thread, exception, Stacktrace::Handle(isolate), false);
 }
 
 void Exceptions::ReThrow(Thread* thread,
                          const Instance& exception,
                          const Instance& stacktrace) {
   // Null object is a valid exception object.
@@ skipping 175 matching lines @@
 
 // Throw JavascriptCompatibilityError exception.
 void Exceptions::ThrowJavascriptCompatibilityError(const char* msg) {
   const Array& exc_args = Array::Handle(Array::New(1));
   const String& msg_str = String::Handle(String::New(msg));
   exc_args.SetAt(0, msg_str);
   Exceptions::ThrowByType(Exceptions::kJavascriptCompatibilityError, exc_args);
 }
 
 }  // namespace dart