Add a leg_only test suite, and make sure to run it in our presubmit script.

tools/test.py

 #!/usr/bin/env python
 #
 # 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.
 #
 
 """Test driver for the Dart project used by continuous build and developers."""
 
 
@@ skipping 475 matching lines @@
       self.AfterRun()
     return result
 
   def Cleanup(self):
     return
 
 
 class TestConfiguration(object):
   """Test configurations give test.py the list of tests, e.g. listing a dir."""
 
-  def __init__(self, context, root):
+  def __init__(self, context, root, flags=[]):
     self.context = context
     self.root = root
+    self.flags = flags
 
   def Contains(self, path, filename):
     """Returns True if the given path regexp matches the passed filename."""
 
     if len(path) > len(filename):
       return False
     for i in xrange(len(path)):
       try:
         if not path[i].match(filename[i]):
           return False
@@ skipping 1010 matching lines @@
     for entry in timed_tests[:20]:
       t = FormatTime(entry.duration)
       print '%4i (%s) %s' % (index, t, entry.GetLabel())
       index += 1
 
   return result
 
 
 if __name__ == '__main__':
   sys.exit(Main())