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

runtime/vm/bitfield_test.cc

 // 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.
 
 #include "platform/assert.h"
 #include "vm/globals.h"
 #include "vm/bitfield.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
 UNIT_TEST_CASE(BitFields) {
-  class TestBitFields : public BitField<int32_t, 1, 8> {};
+  class TestBitFields : public BitField<uword, int32_t, 1, 8> {};
   EXPECT(TestBitFields::is_valid(16));
   EXPECT(!TestBitFields::is_valid(256));
   EXPECT_EQ(0x00ffU, TestBitFields::mask());
   EXPECT_EQ(0x001feU, TestBitFields::mask_in_place());
   EXPECT_EQ(1, TestBitFields::shift());
   EXPECT_EQ(8, TestBitFields::bitsize());
   EXPECT_EQ(32U, TestBitFields::encode(16));
   EXPECT_EQ(16, TestBitFields::decode(32));
   EXPECT_EQ(2U, TestBitFields::update(1, 16));
 }
 
 }  // namespace dart