Run x64 dart2js version by default.

sdk/bin/dart2js

 #!/bin/bash
 # 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.
 
 function follow_links() {
   file="$1"
   while [ -h "$file" ]; do
     # On Mac OS, readlink -f doesn't work.
     file="$(readlink "$file")"
@@ skipping 36 matching lines @@
   read -a OPTIONS <<< "$DART_VM_OPTIONS"
   EXTRA_VM_OPTIONS+=("${OPTIONS[@]}")
 fi
 
 DART_ROOT="$(cd "${SDK_DIR}/.." ; pwd -P)"
 
 DART2JS="$DART_ROOT/pkg/compiler/lib/src/dart2js.dart"
 
 if [ -z "$DART_CONFIGURATION" ];
 then
-  DART_CONFIGURATION="ReleaseIA32"
+  DART_CONFIGURATION="ReleaseX64"
 fi
 
 if [[ `uname` == 'Darwin' ]]; then
   BUILD_DIR="$DART_ROOT/xcodebuild/$DART_CONFIGURATION"
 else
   BUILD_DIR="$DART_ROOT/out/$DART_CONFIGURATION"
 fi
 
 PACKAGE_ROOT="$BUILD_DIR/packages/"
 
 exec "$DART" "${EXTRA_VM_OPTIONS[@]}" "--package-root=$PACKAGE_ROOT" "$DART2JS" "${EXTRA_OPTIONS[@]}" "$@"