Fixes crashes in VM isolate shutdown.

runtime/vm/compiler.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 "vm/compiler.h"
 
 #include "vm/assembler.h"
 
 #include "vm/ast_printer.h"
 #include "vm/block_scheduler.h"
@@ skipping 1023 matching lines @@
     per_compile_timer.Stop();
 
     if (FLAG_trace_compiler) {
       ISL_Print("--> '%s' entry: %#" Px " size: %" Pd " time: %" Pd64 " us\n",
                 function.ToFullyQualifiedCString(),
                 Code::Handle(function.CurrentCode()).EntryPoint(),
                 Code::Handle(function.CurrentCode()).Size(),
                 per_compile_timer.TotalElapsedTime());
     }
 
+    ASSERT(isolate->debugger() != NULL);
     isolate->debugger()->NotifyCompilation(function);
 
     if (FLAG_disassemble && FlowGraphPrinter::ShouldPrint(function)) {
       DisassembleCode(function, optimized);
     } else if (FLAG_disassemble_optimized &&
                optimized &&
                FlowGraphPrinter::ShouldPrint(function)) {
       // TODO(fschneider): Print unoptimized code along with the optimized code.
       ISL_Print("*** BEGIN CODE\n");
       DisassembleCode(function, true);
@@ skipping 275 matching lines @@
     const Object& result =
       PassiveObject::Handle(isolate->object_store()->sticky_error());
     isolate->object_store()->clear_sticky_error();
     return result.raw();
   }
   UNREACHABLE();
   return Object::null();
 }
 
 }  // namespace dart