Generate snapshot for dartj2s; use it in the SDK.

dart/utils/compiler/build_helper.dart

 // 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.
 
 #import('dart:io');
 #import('dart:uri');
 
 #import('../../lib/compiler/implementation/util/uri_extras.dart');
 #import('../../lib/compiler/implementation/filenames.dart');
 
@@ skipping 98 matching lines @@
 BIN_DIR="\$(cd "\${0%/*}" ; pwd -P)"
 
 unset COLORS
 if test -t 1; then
   # Stdout is a terminal.
   if test 8 -le `tput colors`; then
     # Stdout has at least 8 colors, so enable colors.
     COLORS="--enable-diagnostic-colors"
   fi
 fi
-exec "\$BIN_DIR"/dart$options "\$BIN_DIR/$path" \$COLORS "\$@"
+
+unset SNAPSHOT
+if test -f "\$BIN_DIR/${path}.snapshot"; then
+  # TODO(ahe): Remove the following line when we are relatively sure it works.
+  echo Using snapshot "\$BIN_DIR/${path}.snapshot" 1>&2
+  SNAPSHOT="--use_script_snapshot=\$BIN_DIR/${path}.snapshot"
+fi
+exec "\$BIN_DIR"/dart$options \$SNAPSHOT "\$BIN_DIR/$path" \$COLORS "\$@"
 ''',
 '''
 @echo off
 REM Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 REM for details. All rights reserved. Use of this source code is governed by a
 REM BSD-style license that can be found in the LICENSE file.
 
 set SCRIPTPATH=%~dp0
 
 REM Does the path have a trailing slash? If so, remove it.
 if %SCRIPTPATH:~-1%==\ set SCRIPTPATH=%SCRIPTPATH:~0,-1%
 
 set arguments=%*
 
 "%SCRIPTPATH%\dart.exe"$options "%SCRIPTPATH%$pathWin" %arguments%
 '''.replaceAll('\n', '\r\n')];
 }