Use a trail instead of a mark bit when processing recursive types in the VM

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 31270)
+++ runtime/vm/raw_object.h	(working copy)
@@ -1191,7 +1191,6 @@
   RawObject** to() {
     return reinterpret_cast<RawObject**>(&ptr()->type_);
   }
-  bool is_being_checked_;  // Transient field, not snapshotted.
 };
 
 
@@ -1225,7 +1224,6 @@
   RawObject** to() {
     return reinterpret_cast<RawObject**>(&ptr()->type_parameter_);
   }
-  bool is_being_checked_;  // Transient field, not snapshotted.
 };