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

runtime/vm/native_arguments.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_NATIVE_ARGUMENTS_H_
 #define VM_NATIVE_ARGUMENTS_H_
 
 #include "platform/assert.h"
 #include "platform/memory_sanitizer.h"
 #include "vm/globals.h"
@@ skipping 171 matching lines @@
     kInstanceFunctionBit = 1,
     kClosureFunctionBit = 2,
   };
   enum ArgcTagBits {
     kArgcBit = 0,
     kArgcSize = 24,
     kFunctionBit = 24,
     kFunctionSize = 2,
     kAutoSetupScopeBit = 26,
   };
-  class ArgcBits : public BitField<int, kArgcBit, kArgcSize> {};
-  class FunctionBits : public BitField<int, kFunctionBit, kFunctionSize> {};
-  class AutoSetupScopeBits : public BitField<int, kAutoSetupScopeBit, 1> {};
+  class ArgcBits : public BitField<intptr_t, int32_t, kArgcBit, kArgcSize> {};
+  class FunctionBits :
+      public BitField<intptr_t, int, kFunctionBit, kFunctionSize> {};
+  class AutoSetupScopeBits :
+      public BitField<intptr_t, int, kAutoSetupScopeBit, 1> {};
   friend class Api;
   friend class BootstrapNatives;
   friend class Simulator;
 
   // Since this function is passed a RawObject directly, we need to be
   // exceedingly careful when we use it.  If there are any other side
   // effects in the statement that may cause GC, it could lead to
   // bugs.
   void SetReturnUnsafe(RawObject* value) const {
     *retval_ = value;
@@ skipping 14 matching lines @@
     // In the instance closure function case, the receiver is accessed from
     // the context and the closure at index 0 is hidden, so the apparent
     // argument count remains unchanged.
     if (function_bits == kClosureFunctionBit) {
       return 1;
     }
     return 0;
   }
 
   Thread* thread_;  // Current thread pointer.
-  int argc_tag_;  // Encodes argument count and invoked native call type.
+  intptr_t argc_tag_;  // Encodes argument count and invoked native call type.
   RawObject*(*argv_)[];  // Pointer to an array of arguments to runtime call.
   RawObject** retval_;  // Pointer to the return value area.
 };
 
 }  // namespace dart
 
 #endif  // VM_NATIVE_ARGUMENTS_H_