Fixes a couple problems with GC of unoptimized code.

runtime/vm/object.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/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 4093 matching lines @@
   StorePointer(&raw_ptr()->code_, value.raw());
   ASSERT(Function::Handle(value.function()).IsNull() ||
     (value.function() == this->raw()));
   value.set_function(*this);
 }
 
 
 void Function::DetachCode() const {
   // Set unoptimized code as non-entrant, and set code and unoptimized code
   // to null.
+  ASSERT(!HasOptimizedCode());
   CodePatcher::PatchEntry(Code::Handle(unoptimized_code()));
   StorePointer(&raw_ptr()->code_, Code::null());
   StorePointer(&raw_ptr()->unoptimized_code_, Code::null());
 }
 
 
 void Function::ReattachCode(const Code& code) const {
+  ASSERT(CurrentCode() == Code::null());
+  ASSERT(unoptimized_code() == Code::null());
+  ASSERT(!code.IsNull());
+  ASSERT(!code.is_optimized());
   StorePointer(&raw_ptr()->code_, code.raw());
   StorePointer(&raw_ptr()->unoptimized_code_, code.raw());
   CodePatcher::RestoreEntry(code);
 }
 
 
 void Function::SwitchToUnoptimizedCode() const {
   ASSERT(HasOptimizedCode());
 
   const Code& current_code = Code::Handle(CurrentCode());
@@ skipping 11633 matching lines @@
   return "_MirrorReference";
 }
 
 
 void MirrorReference::PrintToJSONStream(JSONStream* stream, bool ref) const {
   JSONObject jsobj(stream);
 }
 
 
 }  // namespace dart