Fixes crashes in VM isolate shutdown.

runtime/vm/object_test.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 "platform/globals.h"
 
 #include "vm/assembler.h"
 #include "vm/class_finalizer.h"
 #include "vm/dart_api_impl.h"
 #include "vm/dart_entry.h"
@@ skipping 4164 matching lines @@
   const String& name = String::Handle(String::New(TestCase::url()));
   const Library& vmlib = Library::Handle(Library::LookupLibrary(name));
   EXPECT(!vmlib.IsNull());
   const Class& class_a = Class::Handle(
       vmlib.LookupClass(String::Handle(Symbols::New("A"))));
   const Function& func_b =
       Function::Handle(GetFunction(class_a, "b"));
   EXPECT(func_b.CanBeInlined());
 
   // After setting a breakpoint in a function A.b, it is no longer inlineable.
+  ASSERT(Isolate::Current()->debugger() != NULL);
   Breakpoint* bpt =
       Isolate::Current()->debugger()->SetBreakpointAtLine(name,
                                                           kBreakpointLine);
   ASSERT(bpt != NULL);
   EXPECT(!func_b.CanBeInlined());
 }
 
 
 TEST_CASE(SpecialClassesHaveEmptyArrays) {
   ObjectStore* object_store = Isolate::Current()->object_store();
@@ skipping 548 matching lines @@
   EXPECT(iterator.MoveNext());
   object = iterator.CurrentKey();
   EXPECT_STREQ("z", object.ToCString());
   object = iterator.CurrentValue();
   EXPECT_STREQ("5", object.ToCString());
 
   EXPECT(!iterator.MoveNext());
 }
 
 }  // namespace dart