Fixes for checked mode and change buildbot script to run dart2js tests in checked mode also.

utils/compiler/buildbot.py

 #!/usr/bin/python
 
 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Dart frog buildbot steps
 
 Runs tests for the frog or dart2js compiler.
 """
@@ skipping 259 matching lines @@
     print '@@@STEP_FAILURE@@@'
     return status
 
   test_flags = []
   if shard_index:
     test_flags = ['--shards=%s' % total_shards, '--shard=%s' % shard_index]
 
   # First we run all the regular tests.
   status = TestCompiler(compiler, runtime, mode, system, option, test_flags)
 
-  # BUG(3281): We do not run checked mode tests on dart2js.
-  if status == 0 and compiler != 'dart2js':
+  # We only run checked mode tests when the host is not in checked mode.
+  if status == 0 and option != 'checked' and runtime == 'd8':
     status = TestCompiler(compiler, runtime, mode, system, option,
                           test_flags + ['--checked'])
 
   if runtime != 'd8': CleanUpTemporaryFiles(system, runtime)
   if status != 0: print '@@@STEP_FAILURE@@@'
   return status
 
 if __name__ == '__main__':
   sys.exit(main())