Try to better guess Visual Studio path when building Dart on Windows.

tools/build.py

 #!/usr/bin/env python
 #
 # 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 optparse
 import os
 import shutil
@@ skipping 22 matching lines @@
       metavar='[all,ia32,x64,simarm,arm,simmips,mips]',
       default=utils.GuessArchitecture())
   result.add_option("--os",
     help='Target OSs (comma-separated).',
     metavar='[all,host,android]',
     default='host')
   result.add_option("-j",
       help='The number of parallel jobs to run.',
       metavar=HOST_CPUS,
       default=str(HOST_CPUS))
+  (vs_directory, vs_executable) = utils.GuessVisualStudioPath()
   result.add_option("--devenv",
       help='Path containing devenv.com on Windows',
-      default='C:\\Program Files (x86)\\Microsoft Visual Studio 10.0\\Common7'
-      '\\IDE')
+      default=vs_directory)
   result.add_option("--executable",
       help='Name of the devenv.com/msbuild executable on Windows (varies for '
-      'different versions of Visual Studio)', default='devenv.com')
+           'different versions of Visual Studio)',
+      default=vs_executable)
   return result
 
 
 def ProcessOsOption(os):
   if os == 'host':
     return HOST_OS
   return os
 
 
 def ProcessOptions(options, args):
@@ skipping 259 matching lines @@
         process.wait()
         if process.returncode != 0:
           print "BUILD FAILED"
           return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(Main())