Fixes crashes in VM isolate shutdown.

runtime/vm/code_generator.cc

 // Copyright (c) 2013, 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/code_generator.h"
 
 #include "vm/assembler.h"
 #include "vm/ast.h"
 #include "vm/code_patcher.h"
 #include "vm/compiler.h"
@@ skipping 680 matching lines @@
 static void CheckResultError(const Object& result) {
   if (result.IsError()) {
     Exceptions::PropagateError(Error::Cast(result));
   }
 }
 
 
 // Gets called from debug stub when code reaches a breakpoint
 // set on a runtime stub call.
 DEFINE_RUNTIME_ENTRY(BreakpointRuntimeHandler, 0) {
-  ASSERT(isolate->debugger() != NULL);
   DartFrameIterator iterator;
   StackFrame* caller_frame = iterator.NextFrame();
   ASSERT(caller_frame != NULL);
   uword orig_stub =
       isolate->debugger()->GetPatchedStubAddress(caller_frame->pc());
   isolate->debugger()->SignalBpReached();
   ASSERT((orig_stub & kSmiTagMask) == kSmiTag);
   arguments.SetReturn(Smi::Handle(reinterpret_cast<RawSmi*>(orig_stub)));
 }
 
 
 DEFINE_RUNTIME_ENTRY(SingleStepHandler, 0) {
-  ASSERT(isolate->debugger() != NULL);
   isolate->debugger()->DebuggerStepCallback();
 }
 
 
 // An instance call of the form o.f(...) could not be resolved.  Check if
 // there is a getter with the same name.  If so, invoke it.  If the value is
 // a closure, invoke it with the given arguments.  If the value is a
 // non-closure, attempt to invoke "call" on it.
 static bool ResolveCallThroughGetter(const Instance& receiver,
                                      const Class& receiver_class,
@@ skipping 959 matching lines @@
   const intptr_t elm_size = old_data.ElementSizeInBytes();
   const TypedData& new_data =
       TypedData::Handle(TypedData::New(cid, new_size, Heap::kOld));
   TypedData::Copy(new_data, 0, old_data, 0, old_size * elm_size);
   typed_data_cell.SetAt(0, new_data);
   arguments.SetReturn(new_data);
 }
 
 
 }  // namespace dart