Fix build failure on linux.

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 "vm/globals.h"
 #include "vm/stub_code.h"
@@ skipping 75 matching lines @@
       return 1;
     }
     return 0;
   }
 
   int NativeArgCount() const {
     return ArgCount() - NumHiddenArgs();
   }
 
   RawObject* NativeArgAt(int index) const {
-    const int num_hidden_args = NumHiddenArgs();
-    const int actual_index = index + num_hidden_args;
-    ASSERT((actual_index >= num_hidden_args) && (index < ArgCount()));
+    ASSERT((index >= 0) && (index < NativeArgCount()));
     if ((index == 0) && ToClosureFunction() && ToInstanceFunction()) {
       // Retrieve the receiver from the context.
       const Context& context = Context::Handle(isolate_->top_context());
       return context.At(0);
     } else {
+      const int actual_index = index + NumHiddenArgs();
       return ArgAt(actual_index);
     }
   }
 
   void SetReturn(const Object& value) const;
 
   static intptr_t isolate_offset() {
     return OFFSET_OF(NativeArguments, isolate_);
   }
   static intptr_t argc_tag_offset() {
@@ skipping 48 matching lines @@
 
   Isolate* isolate_;  // Current isolate pointer.
   int 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_