Give up on using va_copy for now and get the build working.

runtime/vm/dart_api_impl.h

 // Copyright (c) 2011, 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_DART_API_IMPL_H_
 #define VM_DART_API_IMPL_H_
 
 #include "vm/allocation.h"
 
 namespace dart {
@@ skipping 19 matching lines @@
   // Validates and converts the passed in handle as a persistent handle.
   static PersistentHandle* UnwrapAsPersistentHandle(const ApiState& state,
                                                     Dart_Handle object);
 
   // Gets the handle used to designate successful return.
   static Dart_Handle Success();
 
   // Generates a handle used to designate an error return.
   static Dart_Handle Error(const char* format, ...);
 
-  static Dart_Handle VError(const char* format, va_list args);
-
   // Gets a handle to Null.
   static Dart_Handle Null();
 
   // Gets a handle to True.
   static Dart_Handle True();
 
   // Gets a handle to False
   static Dart_Handle False();
 
   // Allocates space in the local zone.
   static uword Allocate(intptr_t size);
 
   // Reallocates space in the local zone.
   static uword Reallocate(uword ptr, intptr_t old_size, intptr_t new_size);
 };
 
 }  // namespace dart.
 
 #endif  // VM_DART_API_IMPL_H_