Change default build and test architecture to x64.

tools/testing/dart/test_options.dart

 // Copyright (c) 2013, 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.
 
 library test_options_parser;
 
 import "dart:io";
 import "drt_updater.dart";
 import "test_suite.dart";
 import "path.dart";
@@ skipping 89 matching lines @@
                'chrome', 'safari', 'ie9', 'ie10', 'ie11', 'opera',
                'chromeOnAndroid', 'safarimobilesim',
                'ContentShellOnAndroid', 'DartiumOnAndroid', 'none'],
               'vm'),
           new _TestOptionSpecification(
               'arch',
               'The architecture to run tests for',
               ['-a', '--arch'],
               ['all', 'ia32', 'x64', 'arm', 'armv5te', 'arm64', 'mips',
                'simarm', 'simarmv5te', 'simarm64', 'simmips'],
-              'ia32'),
+              'x64'),
           new _TestOptionSpecification(
               'system',
               'The operating system to run tests on',
               ['-s', '--system'],
               ['linux', 'macos', 'windows'],
               Platform.operatingSystem),
           new _TestOptionSpecification(
               'checked',
               'Run tests in checked mode',
               ['--checked'],
@@ skipping 765 matching lines @@
         return option;
       }
     }
     print('Unknown test option $name');
     exit(1);
   }
 
 
   List<_TestOptionSpecification> _options;
 }