Background compilation work:

runtime/vm/heap.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/heap.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/flags.h"
 #include "vm/isolate.h"
@@ skipping 161 matching lines @@
     ASSERT(space == kOld);
     old_space_.AllocateExternal(size);
     if (old_space_.NeedsGarbageCollection()) {
       CollectAllGarbage();
     }
   }
 }
 
 void Heap::FreeExternal(intptr_t size, Space space) {
   if (space == kNew) {
-    isolate()->AssertCurrentThreadIsMutator();
     new_space_.FreeExternal(size);
   } else {
     ASSERT(space == kOld);
     old_space_.FreeExternal(size);
   }
 }
 
 void Heap::PromoteExternal(intptr_t size) {
   new_space_.FreeExternal(size);
   old_space_.AllocateExternal(size);
@@ skipping 615 matching lines @@
   Dart::vm_isolate()->heap()->WriteProtect(false, include_code_pages_);
 }
 
 
 WritableVMIsolateScope::~WritableVMIsolateScope() {
   ASSERT(Dart::vm_isolate()->heap()->UsedInWords(Heap::kNew) == 0);
   Dart::vm_isolate()->heap()->WriteProtect(true, include_code_pages_);
 }
 
 }  // namespace dart