Remove zapping of registers that has not turned up any GC bugs in a long time.

runtime/vm/raw_object.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_RAW_OBJECT_H_
 #define VM_RAW_OBJECT_H_
 
 #include "platform/assert.h"
 #include "vm/atomic.h"
 #include "vm/globals.h"
@@ skipping 167 matching lines @@
 };
 
 enum {
   kSmiTag = 0,
   kHeapObjectTag = 1,
   kSmiTagSize = 1,
   kSmiTagMask = 1,
   kSmiTagShift = 1,
 };
 
-enum {
-  kInvalidObjectPointer = kHeapObjectTag,
-};
-
 enum TypedDataElementType {
 #define V(name) k##name##Element,
 CLASS_LIST_TYPED_DATA(V)
 #undef V
 };
 
 #define SNAPSHOT_WRITER_SUPPORT()                                              \
   void WriteTo(                                                                \
       SnapshotWriter* writer, intptr_t object_id, Snapshot::Kind kind);        \
   friend class SnapshotWriter;                                                 \
@@ skipping 1959 matching lines @@
   COMPILE_ASSERT(kExternalTypedDataInt8ArrayCid ==
                  kTypedDataInt8ArrayViewCid + 15);
   COMPILE_ASSERT(kByteBufferCid == kExternalTypedDataInt8ArrayCid + 14);
   COMPILE_ASSERT(kNullCid == kByteBufferCid + 1);
   return (kNullCid - kTypedDataInt8ArrayCid);
 }
 
 }  // namespace dart
 
 #endif  // VM_RAW_OBJECT_H_