Make presubmit on Dart files check for default value separator.

This should prevent new code using old syntax.

Change-Id: I0620369a34b3838fb4555f5264c93bccf92acc42
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/256213
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>

PRESUBMIT.py

@@ -125,6 +125,10 @@ def _CheckDartFormat(input_api, output_api):
         print('WARNING: dart not found: %s' % (dart))
         return []
 
+    dartFixes = [
+        '--fix-named-default-separator',
+    ]
+
     def HasFormatErrors(filename: str = None, contents: str = None):
         # Don't look for formatting errors in multitests. Since those are very
         # sensitive to whitespace, many cannot be reformatted without breaking
@@ -138,6 +142,7 @@ def _CheckDartFormat(input_api, output_api):
         args = [
             dart,
             'format',
+        ] + dartFixes + [
             '--set-exit-if-changed',
             '--output=none',
             '--summary=none',
@@ -181,8 +186,8 @@ def _CheckDartFormat(input_api, output_api):
             output_api.PresubmitError(
                 'File output does not match dart format.\n'
                 'Fix these issues with:\n'
-                '%s format %s%s' %
-                (dart, lineSep, lineSep.join(unformatted_files)))
+                '%s format %s%s%s' % (dart, ' '.join(dartFixes), lineSep,
+                                      lineSep.join(unformatted_files)))
         ]
 
     return []