Fixes crashes in VM isolate shutdown.

runtime/vm/parser_test.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/ast_printer.h"
 #include "vm/class_finalizer.h"
 #include "vm/debugger.h"
 #include "vm/longjump.h"
 #include "vm/object.h"
@@ skipping 186 matching lines @@
 
   output[0] = '\0';
   return output_buffer;
 }
 
 
 // Saves the var descriptors for all frames on the stack as a string.
 static void SaveVars(Dart_IsolateId isolate_id,
                      intptr_t bp_id,
                      const Dart_CodeLocation& loc) {
+  ASSERT(Isolate::Current()->debugger() != NULL);
   DebuggerStackTrace* stack =
       Isolate::Current()->debugger()->StackTrace();
   intptr_t num_frames = stack->Length();
   const int kBufferLen = 2048;
   char* buffer = new char[kBufferLen];
   char* pos = buffer;
   LocalVarDescriptors& var_desc = LocalVarDescriptors::Handle();
   for (intptr_t i = 0; i < num_frames; i++) {
     ActivationFrame* frame = stack->FrameAt(i);
     var_desc = frame->code().GetLocalVarDescriptors();
@@ skipping 342 matching lines @@
       "a\n"
       " 0 CurrentCtx    scope=0   begin=0   end=0"
       "   name=:current_context_var\n"
       " 1 ContextLevel  level=1   scope=2   begin=3   end=79\n"
       " 2 ContextVar    level=1   begin=9   end=79  name=x\n"
       " 3 StackVar      scope=2   begin=11  end=79  name=b\n",
       CaptureVarsAtLine(lib, "a", 10));
 }
 
 }  // namespace dart