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

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index a71d7d55e1b98c03faced6e44430137da5b3768d..45d494c6c14f85de62bb1401c3103338145742fe 100644
--- a/PRESUBMIT.py
+++ b/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 []