Do not reuse tags_ field to store next_ pointer of FreeListElement.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index a62bbd0955500234594b75ca51c56adbb9239189..460c7c641bfec10957b421dc628004f83bad8924 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -189,6 +189,8 @@ class RawObject {
     kClassIdTagSize = 16
   };
 
+  class FreeBit : public BitField<bool, kFreeBit, 1> {};
+
   // Encodes the object size in the tag in units of object alignment.
   class SizeTag {
    public:
@@ -272,12 +274,12 @@ class RawObject {
   intptr_t Size() const {
     uword tags = ptr()->tags_;
     intptr_t result = SizeTag::decode(tags);
-    if ((result != 0) && !FreeBit::decode(tags)) {
+    if (result != 0) {
       ASSERT(result == SizeFromClass());
       return result;
     }
     result = SizeFromClass();
-    ASSERT((result > SizeTag::kMaxSizeTag) || FreeBit::decode(tags));
+    ASSERT(result > SizeTag::kMaxSizeTag);
     return result;
   }
 
@@ -318,8 +320,6 @@ class RawObject {
   uword tags_;  // Various object tags (bits).
 
  private:
-  class FreeBit : public BitField<bool, kFreeBit, 1> {};
-
   class MarkBit : public BitField<bool, kMarkBit, 1> {};
 
   class CanonicalObjectTag : public BitField<bool, kCanonicalBit, 1> {};
@@ -336,8 +336,6 @@ class RawObject {
 
   intptr_t GetClassId() const {
     uword tags = ptr()->tags_;
-    // TODO(vegorov): stop destroying tags_ when creating FreeListElement.
-    ASSERT(!FreeBit::decode(tags));
     return ClassIdTag::decode(tags);
   }