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

runtime/vm/native_arguments.h

Index: runtime/vm/native_arguments.h
diff --git a/runtime/vm/native_arguments.h b/runtime/vm/native_arguments.h
index 028b58b5f34165af6fd4ee58653077fa0251a11c..8bb1d6e0ccdeddb4b3425b75f5e02f654bbc73a3 100644
--- a/runtime/vm/native_arguments.h
+++ b/runtime/vm/native_arguments.h
@@ -189,9 +189,11 @@ class NativeArguments {
     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;
@@ -226,7 +228,7 @@ class NativeArguments {
   }
 
   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.
 };