Add Dart_GetNativeArgumentCount() and a test.

runtime/include/dart_api.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 INCLUDE_DART_API_H_
 #define INCLUDE_DART_API_H_
 
 #ifdef __cplusplus
 #define DART_EXTERN_C extern "C"
 #else
@@ skipping 336 matching lines @@
                                            int number_of_arguments,
                                            Dart_Handle* arguments);
 DART_EXPORT Dart_Result Dart_InvokeClosure(Dart_Handle closure,
                                            int number_of_arguments,
                                            Dart_Handle* arguments);
 
 
 // Interaction with native methods.
 DART_EXPORT Dart_Handle Dart_GetNativeArgument(Dart_NativeArguments args,
                                                int index);
+DART_EXPORT int Dart_GetNativeArgumentCount(Dart_NativeArguments args);
 DART_EXPORT void Dart_SetReturnValue(Dart_NativeArguments args,
                                      Dart_Handle retval);
 
 // Library.
 DART_EXPORT bool Dart_IsLibrary(Dart_Handle object);
 DART_EXPORT Dart_Result Dart_LibraryUrl(Dart_Handle library);
 DART_EXPORT Dart_Result Dart_LibraryImportLibrary(Dart_Handle library,
                                                   Dart_Handle import);
 
 DART_EXPORT Dart_Result Dart_LookupLibrary(Dart_Handle url);
@@ skipping 68 matching lines @@
 // External pprof support for gathering and dumping symbolic information
 // that can be used for better profile reports for dynamically generated
 // code.
 DART_EXPORT void Dart_InitPprofSupport();
 DART_EXPORT void Dart_GetPprofSymbolInfo(void** buffer, int* buffer_size);
 
 // Check set vm flags.
 DART_EXPORT bool Dart_IsVMFlagSet(const char* flag_name);
 
 #endif  // INCLUDE_DART_API_H_