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

runtime/vm/flow_graph_optimizer.cc

 // Copyright (c) 2013, 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 "vm/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/compiler.h"
 #include "vm/cpu.h"
@@ skipping 5804 matching lines @@
   }
 
   static intptr_t ElementSizeMultiplier(ElementSize size) {
     return 1 << (static_cast<intptr_t>(size) - static_cast<intptr_t>(kInt8));
   }
 
   static intptr_t RoundByteOffset(ElementSize size, intptr_t offset) {
     return offset & ~(ElementSizeMultiplier(size) - 1);
   }
 
-  typedef BitField<Kind, 0, 3> KindBits;
-  typedef BitField<Representation, KindBits::kNextBit, 11> RepresentationBits;
-  typedef BitField<
-      ElementSize, RepresentationBits::kNextBit, 3> ElementSizeBits;
+  class KindBits : public BitField<uword, Kind, 0, 3> {};
+  class RepresentationBits :
+      public BitField<uword, Representation, KindBits::kNextBit, 11> {};
+  class ElementSizeBits :
+      public BitField<uword, ElementSize, RepresentationBits::kNextBit, 3> {};
 
   uword flags_;
   Definition* instance_;
   union {
     intptr_t raw_selector_;
     const Field* field_;
     intptr_t offset_in_bytes_;
     intptr_t index_constant_;
     Definition* index_;
   };
@@ skipping 3007 matching lines @@
 
   // Insert materializations at environment uses.
   for (intptr_t i = 0; i < exits_collector_.exits().length(); i++) {
     CreateMaterializationAt(
         exits_collector_.exits()[i], alloc, *slots);
   }
 }
 
 
 }  // namespace dart