Fix some more shorten-64-to-32 warnings:

runtime/vm/snapshot.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_SNAPSHOT_H_
 #define VM_SNAPSHOT_H_
 
 #include "platform/assert.h"
 #include "vm/allocation.h"
 #include "vm/bitfield.h"
@@ skipping 103 matching lines @@
   kObjectId = 0x3,
 };
 static const int8_t kHeaderTagBits = 2;
 static const int8_t kObjectIdBits = (kBitsPerInt32 - (kHeaderTagBits + 1));
 static const intptr_t kMaxObjectId = (kMaxUint32 >> (kHeaderTagBits + 1));
 static const bool kAsReference = true;
 static const bool kAsInlinedObject = false;
 static const intptr_t kInvalidPatchIndex = -1;
 
 
-class SerializedHeaderTag : public BitField<enum SerializedHeaderType,
-                                            0,
-                                            kHeaderTagBits> {
-};
+class SerializedHeaderTag :
+    public BitField<intptr_t, enum SerializedHeaderType, 0, kHeaderTagBits> {};
 
 
-class SerializedHeaderData : public BitField<intptr_t,
-                                             kHeaderTagBits,
-                                             kObjectIdBits> {
-};
+class SerializedHeaderData :
+    public BitField<intptr_t, intptr_t, kHeaderTagBits, kObjectIdBits> {};
 
 
 enum DeserializeState {
   kIsDeserialized = 0,
   kIsNotDeserialized = 1,
 };
 
 
 enum SerializeState {
   kIsSerialized = 0,
@@ skipping 995 matching lines @@
  private:
   SnapshotWriter* writer_;
   bool as_references_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotWriterVisitor);
 };
 
 }  // namespace dart
 
 #endif  // VM_SNAPSHOT_H_