Fix background compilation issues when creating snapshots.

runtime/vm/heap.h

 // 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.
 
 #ifndef VM_HEAP_H_
 #define VM_HEAP_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/flags.h"
@@ skipping 178 matching lines @@
     SetWeakEntry(raw_obj, kHashes, hash);
   }
   intptr_t GetHash(RawObject* raw_obj) const {
     return GetWeakEntry(raw_obj, kHashes);
   }
   int64_t HashCount() const;
 
   // Associate an id with an object (used when serializing an object).
   // A non-existant id is equal to 0.
   void SetObjectId(RawObject* raw_obj, intptr_t object_id) {
+    ASSERT(Thread::Current()->IsMutatorThread());
     SetWeakEntry(raw_obj, kObjectIds, object_id);
   }
   intptr_t GetObjectId(RawObject* raw_obj) const {
+    ASSERT(Thread::Current()->IsMutatorThread());
     return GetWeakEntry(raw_obj, kObjectIds);
   }
   int64_t ObjectIdCount() const;
   void ResetObjectIdTable();
 
   // Used by the GC algorithms to propagate weak entries.
   intptr_t GetWeakEntry(RawObject* raw_obj, WeakSelector sel) const;
   void SetWeakEntry(RawObject* raw_obj, WeakSelector sel, intptr_t val);
 
   WeakTable* GetWeakTable(Space space, WeakSelector selector) const {
@@ skipping 180 matching lines @@
 // Note: During this scope, the code pages are non-executable.
 class WritableVMIsolateScope : StackResource {
  public:
   explicit WritableVMIsolateScope(Thread* thread);
   ~WritableVMIsolateScope();
 };
 
 }  // namespace dart
 
 #endif  // VM_HEAP_H_