Fix Win64 build of Dart VM.

tools/gyp/configurations_msvs.gypi

 # 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.
 
 {
   'variables': {
     'dart_debug_optimization_level%': '2',
   },
   'target_defaults': {
     'configurations': {
       'Dart_Win_Base': {
         'abstract': 1,
         'defines': [
           '_HAS_EXCEPTIONS=0',  # disable C++ exceptions use in C++ std. libs.
         ],
       },
       'Dart_Win_ia32_Base': {
         'abstract': 1,
       },
       'Dart_Win_x64_Base': {
         'abstract': 1,
+        'msvs_configuration_platform': 'x64',
       },
       'Dart_Win_simarm_Base': {
         'abstract': 1,
       },
       'Dart_Win_simarm64_Base': {
         'abstract': 1,
       },
       'Dart_Win_simmips_Base': {
         'abstract': 1,
       },
@@ skipping 54 matching lines @@
           },
         },
         # C4351 warns MSVC follows the C++ specification regarding array
         # initialization in member initializers.  Code that expects the
         # specified behavior should silence this warning.
         'msvs_disabled_warnings': [4351],
       },
     },
   },
 }