Fix layout tests.

tools/testing/drt-trampoline.py

 # 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.
 #
 # For now we have to use this trampoline to turn --dart-flags command line
 # switch into env variable DART_FLAGS.  Eventually, DumpRenderTree should
 # support --dart-flags and this hack may go away.
 #
 # Expected invocation: python drt-trampoline.py <path to DRT> <DRT command line>
 
@@ skipping 16 matching lines @@
   is_png = False
 
   # parse arguments, filtering out flags, and selecting the input test file
   for arg in command_line:
     if arg.startswith(DART_FLAGS_PREFIX):
       env = dict(os.environ.items())
       env['DART_FLAGS'] = arg[len(DART_FLAGS_PREFIX):]
     elif arg.startswith(OUT_EXPECTATION_PREFIX):
       out_expected_file = arg[len(OUT_EXPECTATION_PREFIX):]
       if out_expected_file.endswith('.png'):
-        cmd.append('--pixel-tests')
         cmd.append('--notree')
         is_png = True
       elif not out_expected_file.endswith('.txt'):
         raise Exception(
             'Bad file expectation (%s), ' % out_expected_file
             + 'please specify either a .txt or a .png file')
     elif arg.endswith('.html'):
       test_file = arg
-      cmd.append(arg)
     else:
       cmd.append(arg)
 
+  if is_png:
+    # pixel tests are specified by running DRT "foo.html'-p"
+    cmd.append(test_file + "'-p")
+  else:
+    cmd.append(test_file)
 
   stdout = subprocess.PIPE if out_expected_file else None
   p = subprocess.Popen(cmd, env=env, stdout=stdout)
   output, error = p.communicate()
   if p.returncode != 0:
     raise Exception('Failed to run command. return code=%s' % p.returncode)
 
   if out_expected_file:
     # Compare output to the given expectation file.
     expectation = None
     if is_png:
       # DRT prints the image to STDOUT, but includes extra text that we trim:
-      # - 4 header lines
+      # - several header lines until a line saying 'Content-Length:'
       # - a '#EOF\n' at the end
-      output = output.replace('\n', '_', 3)
-      output = output[output.find('\n') + 1: -len('#EOF\n')]
+      last_header_line = output.find('Content-Length:')
+      start_pos = output.find('\n', last_header_line) + 1
+      output = output[start_pos : -len('#EOF\n')]
     if os.path.exists(out_expected_file):
       with open(out_expected_file, 'r') as f:
         expectation = f.read()
     else:
       # Instructions on how to create the expectation will be printed below
       # (outout != expectation)
       print 'File %s was not found' % out_expected_file
       expectation = None
 
     # Report test status using the format test.dart expects to see from DRT.
     print 'Content-Type: text/plain'
     if expectation == output:
       print 'PASS'
       print 'Expectation matches'
     else:
       # Generate a temporary file in the same place as the .html file:
       out_file = test_file[:test_file.rfind('.html')] + out_expected_file[-4:]
       with open(out_file, 'w') as f:
         f.write(output)
       print 'FAIL'
-      print 'Expectation didn\'t match. Update expectations by running:\n'
+      print 'Expectation didn\'t match.\n'
+      if len(output) == 0:
+        print ('\033[31mERROR\033[0m: DumpRenderTree generated an empty pixel '
+            'output! This is commonly an error in executing DumpRenderTree, and'
+            ' not that expectations are out of date.\n')
+      print 'You can update expectations by running:\n'
       print 'cp %s %s\n' % (out_file, out_expected_file)
     print '#EOF'
 
 if __name__ == '__main__':
   try:
     sys.exit(main(sys.argv))
   except StandardError as e:
     print 'Fail: ' + str(e)
     sys.exit(1)