Change presumbit to not provide unix-specific syntax on Windows.

Copy-over of https://codereview.chromium.org/3005843002/

Change-Id: I8b95201f0570a249054597fffac89cc848791865
Reviewed-on: https://dart-review.googlesource.com/4341
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>

PRESUBMIT.py

@@ -86,10 +86,14 @@ def _CheckDartFormat(input_api, output_api):
           unformatted_files.append(filename)
 
   if unformatted_files:
+    lineSep = " \\\n"
+    if windows:
+      lineSep = " ^\n";
     return [output_api.PresubmitError(
         'File output does not match dartfmt.\n'
         'Fix these issues with:\n'
-        '%s -w \\\n%s' % (prebuilt_dartfmt, ' \\\n'.join(unformatted_files)))]
+        '%s -w%s%s' % (prebuilt_dartfmt, lineSep,
+            lineSep.join(unformatted_files)))]
 
   return []